It's important for you and your husband not to display any disappointment, frustration or negative responses when your son chooses you to help him. But rather than leaving the bathroom or bedroom when these activities are taking place, I'd suggest that your husband occasionally see what would happen if he merely remained in the room. On occasion, you might also ask your son (in a neutral tone) whether he'd like Daddy to help.
I'd also recommend that your husband begin an activity by himself (drawing with crayons, playing with a toy truck) and then ask your son if he'd like to join him. Being physically affectionate with his son is another way to keep their connection strong. Don't get hung up on the activities that he insists upon doing with you. Look for as many ways to do anything as a threesome and let your husband know that this is a phase and that he will get his fair share of time with his son as he continues to grow and develop.