His size sounds fine to me, despite not seeing him (& it's hard to relate to weights given since I don't weigh my snakes or their prey, unlike many here that do). I've only raised one boa, but it took a long time for her to grow- that's normal & actually better for them. If you imagine his body shape in "cross-section", he should be shaped like a loaf of bread (rounded on top, with straight sides) & not round. Too much food will just make him fat (& at risk for fatty liver disease) not healthier, so just be patient. When my BCI got to 5-6' long, she started growing faster & ultimately ended up over 7.5' in length.

I'll add that if the weaned rats seem too small at this point, you could just feed slightly sooner (by a day or 2) before going to the next size up in prey; snakes don't have schedules in nature, & most get by on far less food than our pets do. You say he looks & acts healthy, so trust what you see. You could also change nothing- 2 more months of slightly small prey really won't matter.

We have others here who raise many boas- hopefully they'll add their thoughts.