I have a cautious 5 year old that is currently in a similar place but had a breakthrough this summer and is now getting up independently behind the boat. I echo a lot of advice to not push your son into it. I struggle with this and have to remind myself constantly not too; it has to be his idea. I feel pretty confident this has helped keep my son coming back, not strictly his success on skis.
He got excited about it partially by going to tournaments and seeing a few other kids ski. There aren't a ton around us but there are a couple that go to some local tournaments that are a year or two ahead of him that he sees do quite a bit more and that is exciting. Plus then give trophy's at the tournament and that gets him really excited to try and ski in the tournament.
What has also helped us so far is finding lots of baby steps. We started with an inflatable trainer in the grass, then in the water, then behind the boat. Then we moved to regular trainer skis that are strapped together and pulled through the grass, then played in the water with skis on to practice putting them on and off and just floating with them on. Playing in the water helped a ton with my son to relieve anxiety of being in the water with skis on. Then pulled him on the boom with my help in the water, then without. Then the handle section on the boom with my help, then without. By then he didn't need my help so we went behind the boat without my help and that is where he is now.