What you need to do is tell him how you feel about marriage and commitment and also tell him that you feel he doesn't communicate with you. Communication is a huge part of a committed relationship and if you don't have that then the relationship most likely will not work unless both parties are willing to communicate and respect each others feelings. If a he isn't ready to settle down and he's made that clear in no uncertain terms. Quite often it's the talk about commitment that leads to a break up. If he is feeling like you are pressuring him, he may decide that it's not worth the trouble. If your boyfriend won't commit don't put him on the spot and demand a marriage proposal, chances are that he'll leave if you do this. One approach that you may want to take if your boyfriend won't commit is to let him have his freedom. This doesn't mean that you should encourage him to date other women, but it does mean that you should make it clear that you want to keep your independence and you encourage him to do the same. Make plans without him on occasion, whether it's a dinner out with co-workers or a week long vacation with your girlfriends. You have to show him through your actions that you aren't looking to tie him down. A man is much more likely to commit if he doesn't feel that it's something you expect of him.
Commitment is a big issue in relationships, usually it's the woman who wants a commitment as the ultimate expression of love and the man is trying to avoid commitment because he feels he will feel trapped and will lose his freedom. If you aren't being treated they way you want it is only because at some level you don't believe you deserve to have it. This is a fairly new relationship and he may feel this way now but at some point he may change his mind but it's up to you to know when enough waiting is enough. He looks at marriage as a failure because he has failed three times at it so he is a little gun shy when it comes to that topic and the fact that he said "probably" means he hasn't given up on the possibility of it but just feels like he may not ever do it. What you are going to have to do is prove to him that if he does marry you that nothing will change and he will be allowed to be his own person and have his "ME" time.
The key is to communicate it in the most rational, sensitive way possible. You NEVER want to give ultimatums. All that will do is cause an argument and create hostility and a negative mindset. If he is unreceptive and this is something that he clearly doesn't want, you have several choices. You can hang in there for a while and see how things develop, or leave. Once again, don't threaten to leave if he doesn't do what you want, that creates an ultimatum and not only will you not get the results you want, but you'll be seen as being controlling. Best thing to do is give yourself some time, give yourself a deadline of how long you are willing to stay, and then gently revisit the issue at that time. If the guy is still unreceptive, you can leave knowing that you're not getting what you want out of the relationship and that you gave yourself the time you needed just to be sure.
If he hasn't been much of a talker then it's more than likely that part of his life will never change and if you are only a weekend girlfriend and things have not changed in 18 months then tell him that you feel like a weekend girlfriend and that you deserve better than what you are getting and then you have to decide if you want to continue to be his part time girlfriend or would you like to move on to someone that considers your feelings and wants you in their life more than part time. He is probably set in his ways and won't change much and doesn't feel he has to change which may be the reason he has been married three times. Although you love and care for him if he isn't willing to give you what you want then it may be time to cut your losses and find someone that wants a commitment and marriage and all of the things you want out of life. Life is too short to wait for someone that may never be ready.