Your mom criticises him in front of you right? So then surely you know what bothers her about him? Why don't you try to show your mom his "good side" - parents are VERY big on first impression and its the little things that count for e.g. what he was wearing, whether he had a firm handshake, what job he does, whether he pulled out the chair for your mom/dad, whether he waited for everyone else to start eating before serving himself etc. all these small things add to make "an impression" which means your mom/dad won't know the guy but will gather a sense of him by the way he carries himself. So what do you do?
Make sure he ticks ALL of their boxes - so make sure he buys your mom flowers on her birthday or sends her a card - the kind of flowers SHE likes, wishes your family on certain holidays (christmas or thanksgiving for example), brings something sweet (like pastries or cupcakes) every time he comes over, parents like manners and if your boyfriend does everything by the book - your parents won't have much to complain about! Give your parents LESS to complain about and MORE to like - so whatever is on their list - make sure he ticks it right away! And stay away from their pet peeves FOR EXAMPLE: If they don't like boys who drink please make sure he doesn't order a jug of Tiger for himself at dinner! Or if they don't like long nails, he doesn't have nails that extend way longer than even yours! If they like the colour red or likes SHIRTS then make sure he wears a nice red shirt

. Its the little things that count! So make sure you at least have them done right

Make him invite her out for dinner, let him treat her with kindness and respect, try not to get too physical (hugging, holding hands) in front of your mom as I am assuming she will be slightly conservative and will not consider such behaviour in front of parents as appropriate!
Make sure he pulls out a chair for her, pours her water/a drink first, speaks with respect and is well-mannered and well-dressed!