Cutting off my son's "father"

My son is 3. His dad comes in and out of his life as he pleases. He'll go a couple months without seeing our son. We've never went to court for custody arrangement or child support. He use to give me $250/month but hasn't given me anything in several months. When he asks me if he can get our son I allow it. I don't keep our son from him. But he rarely asks. When he does he has an excuse and backs out. This happens too often.

So I'm fed up. I have been for a long time but haven't had the courage to do anything about it.

My son has a father figure, my boyfriend. He's raised my son for the past 1.5 years. My son calls him daddy which was his own decision. My boyfriend is his "daddy". However, occasionally my son will ask for his dad and I try to explain to him that he can see his dad when his dad will allow it. It's heartbreaking that my son's bio dad comes around just enough to keep my son hanging on and longing to see his dad again. I want to tell his dad to either be in his life on a regular basis or stay away. What he's doing to our child is doing more harm than good.

I've tried having this talk with him several times. He says he's going to see him more but never does. So this is what I want to do: I want to tell him I'm completely fed up. If he wants to see our son he needs to take me to court and get a custody arrangement lined out. This will show he's serious about being a father if he actually follows through with it. If he doesn't go that route then my son won't have a "dad" that comes and goes as he pleases but he will still have his "daddy"(my boyfriend) for anything and everything he'll ever need.

What are your guys input? Should I follow through with my plan? Should i just keep riding it out?