Help! My son feels disloyal to his dad for liking my boyfriend.

I have been divorced for almost two years now and I have been seeing my boyfriend from awhile. I have taken it very slowly introducing my kids to this guy for all the reasons the books say. He is a good guy and I think the kids are finally starting to warm up to him. The problem I am running into is that their father has made no secret of the fact that he hasn’t had much luck with the ladies and is lonely, especially when the kids are with me. They all (especially the oldest) worry about him and want to be able to help. I am realizing that their loyalty to their dad, and his lack of a regular lady friend, might be keeping them from really warming up to my boyfriend and seeing him for the good guy that he is. Any thoughts on this conundrum?