Can employer demand use of sick/vac days for work related injury? on restricted duties, but suffering in pain!

husband injured his back at work, he was out of work for a few weeks and recieved workman's comp. then he was released back to work on light duty. he has been going to physical therapy 3 times a week for 2 months and has recieved 3 (maximum per year!) cortisone shots and is still in so much pain he can hardly move sometimes! it hurts him to sit or stand to long, his restrictions are no standing for long periods, no heavy lifting, squatting, bending, repetitive motion, or walking long distances. he is a custodian at a local high school and his light duty is cleaning all desks and chairs, walking the campus of the school to different cl rooms etc. his sick and vacation days kicked back in july 1st, and when he wanted to use a sick day last week - because he woke up and could hardly move! - they said he couldn't, he had to take a dock day because he was on work restricton!!! this makes NO sense to us! why can't he use his sick day???