How old are the both of you?
Not to be biased. But at a younger age, there's a higher tendency
for your parents to be more firm in their stands.
At a younger age, take things more easily. Express your interest in
this guy. Be sort of like a 'close friend' 1st. bring him around a
bit. See your parents (or even family?) a couple of times, just
like a friend. When they are more accomodised with him, slowly let
the rope in.
At an age where you deserves more freedom. Let your mum have faith
in your decision. You can try the same trick as above in showing
him around a bit. Ultimately, let your mum understand that whether
she accepts it or not, you'll still be with this guy COS YOU LOVE
HIM (pls only say this when you really mean so yeah? 
). BUT you RESPECT her and hope she'll wish
you well and be with you on your choice of relationship. Majority
of mothers should understand from there on if they can do a little
thinking? 
