Just to add, there is no need to rush him nor is there a need to then rush into a marriage. Sorry, I don't mean to be blunt or hurtful here but I feel it is necessary to say so... especially when your mother, who knows you and the groom-to-be, seems to be saying so. Who are we to say anything different here?
[edit: just one last comment - since he is looking forward to marriage, it is his duty to be ready for it in deen. Your mother can't stop you but I can tell you don't want to disappoint her and having read your replies to other comments I see that you acknowledge that she has a valid point. Just keep praying to Allah (SWT) and encouraging him; if he can't handle it or doesn't want it then maybe its best not to go into it and wait. I speak from experience, wait.]