Blame it on auto correct!
My daughter is the type @redpoodles describes above - secondhand shops, thrift shops. She has an old canvas backpack she found in a army/navy surplus store - canvas, lots of character. She wouldn’t sport a Jansport backpack.
You really need to know your audience when buying a gift. When getting a gift from a boyfriend, receiving a gift isn’t one’s style can be worse than receiving no gift at all. It sends the message that the one you love doesn’t really know you or “get” you.
Great suggestions here and, as people have commented, it’s clear that person preference dictates. If she’s really practical and short on money, I think a backpack, after observing or even asking what she wants, is a great idea. If she’s not but your son has noticed she wears a lot of jewelry or scarves and changes it up daily, those could be nice gifts. If your son isn’t sure about her practicality and/or what she seems to like and wear a lot, he can’t go wrong with a fun experience.
What I really wanted to note is that I think it’s really sweet that your son asked for your advice on this!
If he decides to go for a backpack, I think the Hershel is the safest choice. Stylish and not too expensive. The hipster and non-hipster crowd would both like them. I think Fjallraven Kanken backpacks are still stylish among college students, too.
A Jansport backpack is practical but I wouldn’t recommend it as a gift. Too utilitarian!
My daughter’s favorite gift from a boyfriend was last year, when her boyfriend got her a ukulele. Coincidentally, she got him the exact same thing!
As a young woman, a backpack is nice…if I needed a backpack. Otherwise, it’s another bag I need to cycle through.
I would much prefer something personalized that has to deal with my interests. A landyard from a show I watch on Netflix. A t-shirt that has a funny reference from a video game. Something personal. Funny is nice. It doesn’t have to be the deepest.
I think personalization is key.
EDIT: Can’t go wrong with experiences. Groupon has always been a favorite with my SO.
I thought I’d report on first hand information from my D. It’s her birthday and there is a boy she’s been kinda seeing.
She’s made other plans for her birthday night with girlfriends but this boy brought over a gift.
2 gift cards. One to one of her favorite fast food places and another to lululemon. She was delighted that he remembered her birthday and thought the gift cards were perfect. Not too personal but reflect things she likes.
I think getting her a gift card from the college bookstore is a practical gift. Maybe throw in a t-shirt or something fun, to make it a thoughtful gift as well. The fact that he gets her anything is what counts. It is the thought behind the gift not the gift itself (spoken like a guy that hates to waste money).
All the young women I know love Sephora gift cards.
The young women I know probably never heard of Sephora. I know I’ve never seen a product from that place in our household, tho I am mother to a young 20+ woman.
Ditto Himom.
I know what Sephora is. I, however, would not want a gift card there from my boyfriend for my birthday. But that is just me. I like personal things that are tied to my interests and needs.
But…if anyone is offering. 
My favorite thing a boyfriend gave me was a practical object. Ski poles. The gift was a huge hit because my old ones were clunky and heavy. I now let him pick less practical objects for me, like jewelry. 
When I was dating, I liked flowers and a personal item like an inexpensive but pretty necklace or Tshirt of somewhere significant to me or a nice meal out. A gift card would have been meh.
One BF would make me cassette tapes of him playing the flute and send them to me. I still have them decades later, tho haven’t listened to them since we broke up decades ago. Another BF sent me a tape of him playing the guitar. I still have that too, decades later. Both were excellent musicians but H is somewhat tone deaf.
4 months into a relationship means they are probably quite serious by now. I think a back pack is meh.
Years ago, I got my husband (dating for a couple of months) a now-classic Alessi bottle opener. We use it all the time. It was perfect because it was stylish, not too expensive, and made to last. I think a practical but stylish thing is a nice idea. For my daughter’s graduation, I got her a refillable and trendy Filofax planner, I think it was about $26, and a really nice pen with refillable ink cartridges. Also not too expensive, about $25. I think something stylish that can be used for years to come is a good idea, becasue even if the relationship ends, it’s a gift worth hanging on to.
My daugher and her BF (going on several years now) typically celebrate occasions with a nice dinner out (which is a treat from the usual meals at inexpensive places that college students have) and buy each other something small - a t-shirt from a game or show they like.