I guess lots of cuddles, getting him to talk about it, maybe drawing it would help him with his feelings. And when you do have to leave let him know when you'll be coming back, like after lunch, or his sleep.

My 3yo went thru this and it is hard, I'm starting to see some improvement when she gets 1on1 with me. Bit hard with 2 others but I try :-/

You can go see your cyh nurse or equivalent as they may have other ideas to help you out.