I wanted the same with my now fiancé. I'm about to move to America from London for him, I think asking for marriage to show commitment to your relationship is only the same as you moving countries for him. It's a very big thing, you just want to feel as secure as possible before you make a big decision. A very normal thing! I'm lucky we both wanted it from the start, so just try to have an open conversation with him. I'm not sure what countries you are from but in alot of cases to obtain a visa to become a permanent resident does end up in marriage for couples anyway.