he is the sweetest, nicest person in the world and I know he would be a fun and loving Dad to any kids we have
Much of parenting is doing the housework, doing the appointments (dentist, clubs, playdates), doing the cooking, cleaning, washing, keeping the children clean and so on. That's what part of parenting is, running the household so the children are safe, well stimulated, clean and so on.
He can't do that bit. He can't do housework, indeed I expect he thinks it's your job, or is happy for you to pay someone else to do it. That work will increase exponentially when you have a child, so who is going to do it, if not him? What would be the point of having him as a SAHD if he does no housework at all (not even his 50%), and can't give the children a stimulating environment.
OP you have got together with a sweet guy who is going nowhere in life. Would he be so sweet though if you started treating him like an adult, insisted he do his share of the housework (50%) and the bills? My guess is that he would be moody and even aggressive if pushed to contribute equally. I've seen this before, these type of men are nice and sweet when left to do nothing and contribute little, but once asked to behave like adults, they are often resentful and not nice and sweet at all. When he has a child, will he have to have a day off every two days of parenting as it's all so tiring!
Only you know why you think this set up of you doing everything, paying for everything, except for a 10% contribution by him is enough.
It's not a good set up for having children, and if you do go ahead, then expect to continue the doing everything, paying everything scenario, except you will be tired and exhausted and he will be far less sweet when you ask him to step up to the plate.