A year and a half is no where near long enough to know someone to even consider marriage for most men. Since he already has a child, he knows how hard it is to make a relationship work......which means he will take a little longer then a man with no children. The reality is he has not only himself to think about, but his child as well. While you do want a man you can grow with, you also need a man who is smart enough not to jump into anything without time. Which is very important, if he had already asked you.....I would question why he is willing to tie down so quickly.
Most men will not even consider popping the question until the 2-3 year mark. And those who have already been in a serious relationship, tend to take the full 3 years. The reality is you both have children you must think of first. If you jump into marriage and it does not work out the children are the ones who will be hurt the most, so with that in mind taking your time is the best thing you can do not only for your relationship, but also for your children.
What I would suggest is sitting down with him and letting him know that while your not asking to walk down the isle in 6 months, you are curious on what his viewpoints are on the subject further down the road. Ask him if he is interested in marriage within the next few years if things are still good between you. The reality is if he says no, then you have to decide if this is what you want to accept. More then likely he will be ready, just not as quickly as you may want him to be. It sounds like he is pretty mature, which is good for a long term relationship, the last thing you want to do is jump into marriage with a man who places little value on it. It sounds like he views marriage as a big step, and one he wants to make sure if right for him and his child first.......which is a good indication of what kind of husband and father he will make to you and your child.