22inch rims keep denting and air in tires keep escaping?

[Late-Nite]

My boyfriend has a pontiac grand prix 2007 I think and he has 22s on it but for some reason the tires keep going flat every so often like every three months a tire is flat for a random reason and he puts air in the tire but then it goes flat after filling it with air. We taken 2 tires with the rims to a guy who fixes them, Guy says the rims are dented and air is escaping and we have gotten a second opinion from another place too without telling them wat we thought was wrong... but anyway I was wondering if something like this has happened to someone else who has rims and they keep going flat every one in a while... and if its normal? thanks for ur time to answer