If that's what he has been getting and he's readily accepting each week, you could leave him on that. My suspicion though is that 1 large rat or maybe even 1 jumbo rat every 2 weeks would actually be better for him. But whatever works for both him and you should be fine. Just be flexible.
He won't get bigger by living in a bigger cage. The 4x2x2 is a good size for an adult male boa. Get him outta that 3x3!
Just remember, we can not control just how big these snakes will get. There are some that will get huge even staying on rats all their lives. Nothing you do, short of not feeding him, will garauntee he stays small (and I know you won't not feed him...

).
However, he's 5 years old. He's not going to get much bigger. I don't think you have a thing to worry about. Yes, snakes continue to grow their whole lives - but as adults, they do so at a much slower rate. I can not see a 5yr old boa attaining more than a few more inches unless you start him on rabbits. Just feed him appropriately sized rats and he'll do great!
That sounds amazing!! Be sure to post pics someday when you've got it done!
BTW, I see you have 3 Ibizan Hounds! I have an Ibizan too! I raise German Shepherds, but a friend who's been involved with Ibizans for years got me interested in the breed. We co-bred a fabulous litter and I kept Miss Chloe. If you're interested, check out my website...
http://www.freewebs.com/evenstaribizans/