Answer: Hi Katrina,
I'm sorry to hear about your son's illness, but also glad to hear from you; clearly a caring and involved parent. I hesitate to offer an opinion on your particular situation because I don't know your son well enough (or at all!).
One of the unusual things about mental health is that treatment and things that are healthy and helpful can vary greatly from one person to the next. It is a possibility that the video games are indeed helping your son, but with the facts I have there is no way for me to know. I imagine you have discussed your particular question with some degree of depth with the case worker you mention. Since you are not completely satisfied with what you are hearing, perhaps it would be a good idea to get a second opinion from another professional who knows your son well.
Another thing I wonder about is what your son's feelings are on the topic. Although he is ill, he is old enough to have his own say on the hobbies he chooses. It's definitely not easy to cope with schizoaffective disorder and if this makes him really happy then perhaps it is good.
I hope this is a little helpful.