At 6 yrs old, your male BCI is probably nearly as big as he's going to get. Yes, he will continue to grow, but if he's already 6 yrs old, he probably won't ever get more than 6-6 1/2 ft. But that is average for a male BCI. They usually top out at somewhere between 6-7 ft. The males are smaller than the females. The females get up to around 7-9 ft.

I wouldn't be too worried about his size so long as he's healthy, has good weight relative to his size, and has good muscle tone. If these things are all good, then that's what's important whether he's 5 feet or 10 feet!

You might be able to get a little more size on him by feeding a larger prey item, but you must be very careful doing this! Boa's do NOT do well being overfed (or powerfed). I do not recommend this. Your boa will get as big as he was meant to get if you just keep him on an appropriate size prey item and on a reasonable feeding schedule.