He's seen your worst
The "worst" varies from person to person and whatever that is, he has experienced with you and it has made you and him more confident in each other. It also works the way, if you have seen him at his worst and he has stuck through it.
He is surrounded by married couples
His environment is also a very good indicator that he wants you to be part of his future. Most of his friends are happily married couples and he has good connections with them. It is only natural that he thinks about it too.
Updates you on everything in his life
He is counting on you in his future if you are updated about his life. He makes sure you know everything that is going on in his life because he wants you to know. He makes you feel connected and part of his life by involving you in whatever is going on.
He supports your ambitions/dreams
He wants to see you grow and achieve all your goals. He is supportive and actively involved in the things that make you whole as an individual. He is your partner and confidant and fan.
His life is better with you in it
In a relationship headed for tying the knot, he may tell you or show you that his life is better with you in it. He appreciates you and shows you that the relationship is great for him and both of you.
He casually talks about the future
One of the biggest clues is if talks about the future casually during conversations between you two. He might have his sights on the future of your relationship if he drops future plans into conversations.
He values your opinion
If he seeks your opinion on what he does or experiences, it shows how invested he is in you and the relationship. It also shows that he wants to share important life decisions with you.
There are indicators of a future for couples even before a marriage proposal. Each relationship is different and some people may or may not tick all these signs. Even with such indicators, only the two people in the relationship can determine the trajectory of a relationship.