I agree it will greatly depend on the funeral party.
Two years ago, we were part of the family and our casual dress was the most dressed there. It was July, dh wore a black polo shirt and khaki pants with dress shoes and a belt. Ds wore very nice khaki shorts, a black polo shirt and vans or mens sandals (can't remember), no belt. Dd and I wore black pants, sandals and black and white tops.
Honestly, for me it was pretty shocking. So many people arrived in jean shorts, flip-flops, tank-tops and imo... just whatever they happened to be wearing that day. There was no church service, just a small memorial and luncheon.
I totally understand clothing sensitivity; I always try to purchase all cotton for ds. I also made sure he had a quiet place he could slip away to if he needed some space.
I hope you don't need the new clothes for a long time, but purchasing ahead of time and helping him get used to the clothes is a very good idea.