I know it is difficult more so in the elementary years where Impulse control is so much more limited. My first reaction is why would you go on med holidays when you have these issues? The meds will help with relationships and give him a chance to manage the meltdowns. The meds are there for a reason.
The other thing is that he needs tools to employ when he feels like he is going to blow. A therapist can help identify the tools and you will help him remember to use them. Parties have a lot of triggers. You will just have to attend them. Sorry. No more dropping off and hoping for the best. I looked for a way to help the host mom so I could keep an eye on things. You are there to step in when you see him start to escalate.
Lastly, it gets better by high school. I had to do so much work in the younger years to help her find her niche. We tried every sport, art programs, theater programs, etc to help her figure out what was her passion. We did destination imagination, scouting, summer horse camps, so many things. Turns out it was theater and music. That is her tribe today. Good luck.