It’s not ideal that he lives by himself but as long as they can interact through the bars then that will stop him from getting lonely.
What you are dealing with is the fact that they are teenagers, that brings hormone spikes and can cause issues when bonding.
What you can’t do is put them together for a little while to eat for example, and then separate them and put him back by himself, and then have them meet up again. If that is what you are doing (it sounds as if they are eating together or that him and bean are having some physical interaction), then being put together and then separated again isn’t a good idea, every meeting is a bonding session and it is stressful to them to have it interrupted. I may have misunderstood what you meant when you said they are fine when they eat and don’t fight when eating.
If you are going to have sweet pea and bean live together, then do a neutral territory bonding and ensure a successful bond and then have the two of them live in a cage together permanently.
A Comprehensive Guide to Guinea Pig Boars
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Dominance Behaviours In Guinea Pigs
Bonding and Interaction: Illustrated social behaviours and bonding dynamics
We need to have an idea of their weights over the previous weeks to know whether he is gaining at a good rate. Giving us just his weight next week won’t really help us.
Ultimately though if you have concerns about him, then having him checked by a vet would be a good step to take