You’re right. This is a tough situation because you’re taking the moral high road, and he doesn’t have the strength to do the same thing. In addition to which, this isn’t your battle to wage. It’s his. So, while you want him to do the right thing by his son, and get visitation rights and to pay child support, you can’t make him. The only thing anyone can do, is the mother of his child can order a paternity test, and she can get an order for him to pay child support, but she can’t make him see the child.
There is a strong chance that this child will grow up, learn about his real father, and feel angry and abandoned by his dad. If that’s the case, the child will have every right to be angry, and this will probably scar the kid for life. To know your parent doesn’t want to see you is damaging. This isn’t going to make your fiance feel any better about his decision not to get involved in his son’s life — it’s going to make him feel worse.
But, again, there is nothing you can make him do. This isn’t about you — except to the extent it shows you your fiance’s character, and if you don’t like what you see, now is the time to make a break for it.