I think we have all struggled with multivariate calc! Personally, I would encourage your son not to give up, even if it means he will have to take some classes over again. This is not uncommon, and it shouldn't be considered a failure. Some of the college-level courses are very hard, and for some unknown reason some professors just like to make their students think their courses are harder than they need to be. If he should decide to step out of engineering, I don't think I can advise the right major to step into. That is a conversation I think he will have to have with his faculty advisor. That person should be able to guide him more carefully based upon his own skills and desires as well as the university degree programs that may be best related to what he has already done. Good luck to both of you!