Does anybody chinchilla Drop a lot of that weight in October

So my boy is almost 5 years now and ever since I’ve gotten them one of them is always dropped a lot of their weight in October he would be at 428g ( have gone to the vet about his weight and he is healthy otherwise as weight) to 389g sometimes even down to 344g but give it 2-3 months he is back to normal weight for him. Is there anything I can do to help him or just make sure he is eating as this has been happening for 5 years now and it has really effective him at all.