Is he actually biting hard and breaking the skin, or giving you a short, sharp nip? I ask because the two can sometimes have slightly different "meanings" or "reasons", although there are reasons that apply to both biting and nipping.
If it's a hard bite, breaking the skin, there may be a behavioural problem there - as you say, he's always been a biter to some degree - or there may be some significant irritation or pain leading to his behaviour.
If it is a warning nip - a short, quick lunge towards you and placing teeth on you but not breaking the skin - it sounds more likely to be something along the lines of territorial behaviour. Is he very territorial about his cage, or very protective or dominant over Chestnut? Does he nip you when he's completely away from the cage, or only when you go near the cage? Does he exhibit any other behaviours - rumbling, swaggering, teeth chattering - if he sees you're near the cage, or when you put your hands inside the cage?
There are a few things I would suggest trying, and they have already been suggested. Hard as it will be, given his current behaviour, I think treating him for mites - minimum of two doses Ivermectin - and giving him a bath would be the first place to start. This would rule out any irritation. If he's got no other physical symptoms, you could probably rule out pain - pain from cystitis and pain from bloat would be ones to consider and rule out in particular, each can cause significant pain and distress in guineas.
If you can rule out a physical problem, you could consider if it's a "conditioned behaviour" so to speak...just a bad habit that he's learned affects you.
