A pillow or blanket with pictures. A jigsaw with a picture.
This is probably too unique, but my partner had a small collection of pokemon stuffed toys. He was missing one from a set so I bought it for her to give him.
It was my husbands birthday a couple of weeks ago and I was really struggling too. I ended up getting a voucher for a farm so they can have a day out together x
U could make a card for him with ur sons handprint. Xx