We recently had our 10 year old assessed (again), he's still between 2-3 yrs old socially and mentally, with some verbal capacity of a 5 year old. I've tried the timer routine, my husband doesn't like it, he'll always say "just let "10 yr old" have it." Just frustrating, I think he's inadvertently causing sibling resentment in the long run.

Children on the spectrum are usually 3 to 4 years socially and mentally behind, so if he is your 10...he really is 5 or 6 at most as they get older the gap will close.I am talking like 24 the gap gets smaller

That's tough. I understand what your husband is doing. I found myself doing something similar when my son was young. It took some time but I realized I wasn't doing him any favors by not teaching him to share. Teaching him that was hard. We bought a timer and when there was a dispute we would set the timer for 15 minutes and tell who ever had the toy first that they had 15 minutes left and it was someone else's turn. That worked well. Jonathan needs a warning still when his time is almost up. I usually give him 5 min heads up and he stops playing when he is told.

Also - often when I do bring it up like "our 3 year old" had the toy first my husband almost always says "our 10 yr old gets priorty because he's autistic."