I helped (sewed) my sister with the clothing crisis when her ds crushed his growth plate and had to have surgery.
1st. Easiest & likely the cheapest retail, if he can wear larger sizes (nephew was four) is to check ebay for break away or tear away warmups. There are a few web sites, but they can be pricey. I peeked over at ebay and they had a number of options listed now for larger boys and men
2nd Purchase one or two sizes up and make them yourself. JoAnn's has a good quality soft velcro sold by the yard. The dots are one option, but they are very scratchy. I also bought a snap-maker(?) and did a few pairs with snaps. The snaps were pretty easy to apply, compared to sewing on one inch strips of velcro. Don't try to do an entire leg with a length of velcro; it will seal shut and be a pain in the rear. I can give you directions if you need or want them. I even designed breakaway underwear, but nephew was four and his cast was a full leg with no weight bearing for weeks. (I purchased, but didn't use an old pattern (old, there are no current ones available) that you could borrow, but the sizes are 7-8 or 10-12; I'm guessing they would be too small for your ds.)
3rd. Be creative and or enlist help of anyone you know who sews. Once I finished one pair of pants, the following ones were much easier. I found a youtube video with instructions for tear-away pants and just followed it.
My sister found large, big clothing and socks worked very well for her ds too as he healed.
Prayers for you and your ds.