Aw he's beautiful but gosh, so teeny tiny. Going by his size, I would say he is much younger than 6 weeks but I could be wrong.
Boars are usually separated from mummy and any sisters at 3 weeks as boars are usually sexual mature by that age.
Ideally they shouldn't be re homed until they are at least 6 weeks old
I had a piggy once who was just four weeks old when I re homed her but this was due to her mummy getting fed up with her and the rescue were happy for me to take her as the lady new of my experienced etc.
Before you put him with your existing boy, please make sure you are 100% he is a boy. A baby girl becoming pregnant at that age is very dangerous.
Guinea pigs come in all different shapes and sizes. Is it possible you could weigh him? That may help with being able to age him more. You don't want him to be too light. A healthy boar should weigh around 250g at 3 weeks and more than that before being re homed.
I am tagging some of our more experienced members to help you out regarding his age etc and offer more advise.
@helen105281 @Wiebke @sport_billy @furryfriends Excellent Adventure Sanctuary
When it comes to starting him on new veg, please only introduce one new veg at a time as not to cause tummy upsets and make sure he can't squeeze through the bars of his cage / hutch.
If you have a wire hay rack / metal feeding balls etc please remove it as tiny babies can easily become trapped and injured.
Are your piggies living indoor or outdoor? It's too cold at the moment for baby piggies to be living outside especially when they're not used to it.
Good luck x