This is the problem. He’s devaluing his own work by being afraid to set a fair price for his services.
Since I don’t know your son, I don’t know what his true worth is — but when my daughter moved on from her paid employment and shifted to self-employment, she came to me asking for advice on what she should charge her clients. (She does business management consulting in a creative industry.) She told me what she was thinking of charging for her first contract — and I told her that it wasn’t enough, that whatever she thought she should charge, she should quote twice as much. She got similar advice from other friends. I keep giving her the same advice over and over again – I tell her that the client or employer always has the option to negotiate if the price she quotes is too high. But that doesn’t happen – they always come up with her price and often hire her for additional work. She’s good at what she does & has a terrific network of contacts within her field, plus she has carved out a niche specialty — so that’s not going to work for everyone.
But the point is – when a person is willing to work for low pay, their employer perceives them as being a low-level worker. If the person makes it clear that they are available for part-time or short-term work, the employer is only going to call them on an as-needed basis. And there is never room to negotiate pay upwards for a new job – so if your son is willing to work for $10/hour and someone else wants $20/hour – the employer is going to hire the person they perceive as most capable. But they aren’t going to offer to double the pay rate of the $10 worker even if they choose to hire him over the $20/worker.
It’s good news that your son has some contract work now – but I’d advise that he spend some time consulting with someone who can help him with the negotiation and contracting end of things. Depending on the nature of his creative work, maybe he needs an agent.