At cost, means at cost.
If I was say a lawyer, whose performance you are paying for, and my parents asked me to do say, house conveyancing for, I would charge them costs only.
So I would give them the benefit of my legal training and as a practising lawyer, I would have insurance so I would give them the benefit of that too. However if I had to pay out for reports , I would charge them for it. I would not charge them for my time, my knowledge, my performance.
Applying this to their son, if he is a performer, he should already have insurance, and he is a trained musician (at their expense), so I would given them the benefit of both these things including my music repertoire and special requests. I would however pay out of pocket expenses for my son, i.e. mileage, transport etc. The difficulty here is that his fellow band members are not family and this is how they make a living so they do need to be paid...but maybe have mates rates as I am sure it will be a reciprocal arrangement.
I too would be miffed if my sons immediate response was "but you need to pay", it sounds ungrateful. Its an opportunity for him to show his appreciation for his parents and its a bit entitled that he doesn't naturally take this tac.
I also think its a bit entitled that he doesn't give cards/presents to his parents. Does he give to his siblings and doesn't he realise his parents are left out. This does need to be addressed but not by the parents, by his siblings or other family members or maybe his girlfriend.