I don't think you are overthinking this. I think you need to give it alot more thought.

You need to make it very clear what you do and don't want. It might not even be 'fair' to ask him to donate for a baby but have nothing to do with it. However he may be happy with it this way. Make it clear. Get it in writing or legalised in some way.
If the only reason you don't want him to be in the childs life is in case 'he takes the child away' you need to stop and get a bit real. No one can take the child away from you (unless it's docs and you do something terrible). You don't own a child in any case, they need as many people around them that love them that is humanly possible.

However if he takes the role of god parent so to speak you may well find he might want a bigger role. I imagine it might be hard to see a child growing, knowing that it has half your DNA and even looks like you - and be expected to take a back seat.

And even if he did want a bigger part in the childs life - why would that be so bad?

I think you need to be very careful what your own motives are here, and if you are even being realistic.