Congratulations I hope things go smoothly for you and your partner. He sounds like a lovely and mature man so fingers crossed he takes it well. However, situations like this are sometimes complex, and think of it from his side too.
I was in a similar place to you once, and I can tell you that although we worked out fine it was initially a bumpy road for my partner because he didn't expect a child so early on. The first thing you need to do is decide that you truly want the child yourself, even if he is not in the picture and even if he could in no way contribute to it. If the answer is yes, at least then you are not just having his baby to try play happy families. Also, this lowers your expectation of him so he can make his own mind up about how involved he wants to be. In my opinion "forcing" him is worth nothing and will just cause stress & disappointment down the track. In my case although my partner took the news badly initially, he chose to be 100% involved and we married a year later and are still happily together with more children!
When you address it with him, also take it from his perspective - he thought you were infertile and took your word for it, even though you had never tested out your fertility as your last partner had had a vasectomy. In addition, he is a good father who already has children who - no matter how old they are - may feel very displaced by your news and how short your relationship has been. This may affect his initial response.
I hope he takes it very well and your pregnancy is a miricle which should not be diminished. But also be mindful that he took your word for your infertility and it could be an initial shock. Hope it turns out well!