I’d suggest he focus on the non-math and science subjects. Most kids heading into engineering love the math and science but don’t like the English and history classes so much. If we had to do it again with my daughter (who loved science and math but hated to read), I’d have had her take English as a DE class. She took it as an AP class and did well in the class but not on the AP test (she wasn’t a good test taker). It would have benefited her more to get the credits for English, maybe a history or government class as a DE and then she wouldn’t have had to take them in college.
She was actually a pretty good writer and so did fine taking them in college, but it just would have been easier to have had them out of the way. She had to take math and chemistry and physics in college anyway (just the level she started at depended on her high school classes, and she started at the beginning and did just fine with that path too) but if she could have knocked off English 1 and 2 and some of the other 15 credits required in non-engineering classes, she could have taken more electives and frankly enjoyed them more than English and civics.
It all worked out. Daughter entered college with not one credit for AP or DE courses, needed 131 credits to graduate, and did it in 4 years, 8 straight semesters. She just didn’t have much wiggle room.