I'm going to tell my boyfriend that he has an unhealthy relationship with his son
What about their relationship is unhealthy?
He gets his son everything he wants and I think he's spoiling him
You shouldn't interfere, it's not okay to get between your boyfriend and son's relationship. You should be more supportive.
I feel he should discipline him more often.
Please remember that it is his child not yours. If you have a real problem with it you can break up, it is a really hard position to be in.