First of all, from a patent law point of view, your son is not entitled to anything just because he provided the drawings.
Now, drawings he does are certainly protected by Copyright, but if he gives them to the business owner for free, then they are gone. The question here is:
What should your son get as a reward for creating those drawings?
Which is beyond the scope of this site I fear. I'd suggest you talk this through with him, your friend and the business man. They apparently understand that work should be payed. What kind and amount of pay is a question for negotiation, but patent law does not provide an answer here. And after he got paid for those drawings, that's about it. I don't see a reason he should get more than the pay though as he didn't invent the invention nor did he invest more than the time he got paid for.
Basically, your son can gift those drawings to them or sell them and the pay is whatever both parties agree upon. Legally there might be regulations for working as a student or a minimum wage or something else that applies, but patent law doesn't apply.
He could be credited with a copyright notice, but that would still not entitle him to any further rights from any royalties the patent generates, it would only protect the drawings from beeing copied (may vary per jurisdiction, some require a copyright notice, in some it's implicit).
To adress your concern that he might get used - I don't think somebody creating drawings and getting paid for that is getting used. I'd rather interpret that as standard contractors work. If the pay sounds fair, considering there are professionals doing this kind of work too, your son earns some (extra) money and the business gets the work a little cheaper, I'd say that's a win-win.