Help - Major Hairball problems

My cat Chaos is having major hairball problems this year. I brush him frequently. Unfortunately he only lets me brush his back, not his belly. I'd need a full leather body suit to brush his tummy and thats where he needs it most. He gets fed Science diet hairball formula. I've started using Petromalt again. For a long time he was allergic and it made his face/lips swell up. So far so good, no reactions. He's yurking at least once a month if not more. Can I give him the Petromalt more than once a week? I don't want to mess up his GI track with all the laxative. Should I take him to a groomer and get him shaved? He has short/medium length hair with a dense soft fuzzy undercoat (the ultimate culprit of this problem).
Any suggestions would be greatly appreciated?

Your best bet is to have a groomer strip his belly every four months. You can also add pumpkin to his diet. I add mine to their wet food and they eat it right up.

I have Persians so hairballs are always a problem here.

Thank you Dragonlady for the information. I've made an appointment at the vet to get his belly shaved on Thursday.