Well snakes can go a long time without food and in the wild, I would guess a boa probably eats about once a month or more if he is lucky. As to growing that fast in such a short time, maybe it is just a growth spurt. My pied BP went from about 12" to 3' in just over 4 months but I fed her every 3-4 days as a baby and then backed her off to once a week once she hit 433g. This rule doesn't apply to boas as they are different than pythons but just using the growth as an example. Also as for size, different places sometimes sell rats different size labeled as different. Like the old place I used to buy rats from didn't have pups. Pups were just classified as large fuzzies. Now where I buy my rats from, a small rat is about 5" nose to butt, not counting the tail. Mediums are 6"+ and larges are LARGE. Like my 6' ate a large once and it left a huge lump in her and she went into a food coma for about 4 days. Was too big, that's why with mediums for her, it is perfect, it leaves a small lump for a day or 2. So you have to just use your judgment to rat size but rule of thumb for any boa is you just want a slight lump for a day or 2. You don't want them to have a huge bulge as it can lead to regurgitation in a boa which can screw them up for months. That is why BCC owners feed every 2 weeks even small guys and they feed smaller prey. In the case of boas, larger food is not better. And like I said, I've had my 6' BCI girl Rosey since she was about 22" and now after 2 years of owning her she is 6' and she is overweight. I fed her once a week for about 2 years. I have recently cut her back to once every 3-4 weeks as breeders and other people who have boas forever I've talked to have said she needs to be cut back to once every 3-4 weeks. Boas by nature are a slim snake, they aren't all fat and round like ball pythons or most pythons in general. And boas do better if underfed than overfed. The breeder I got my sunglow from was telling me about how his first boa died after 10 years because it was too fat and he fed it too much. He learned and told me also that he feeds even the little ones every 10 days and the big ones every 3-4 weeks. So bottom line is it is your snake and your decision but I am just telling you what I have been told and what I have learned from keeping boas and pythons. Oh also as for him growing big, he is a male? If you know he is a male, then he will top out at about 6' unless hes some mutant. The best way to know though is find out how large the parents were, that will give you an idea but if that's not possible, then I would guess 6' as that's the normal size for males and females are about 8'.