Welcome to the forum.
We have a thread dedicated to guinea pig behaviour which you might find helpful.
Guinea pigs are prey animals and so they hide from anyone who could be a predator.
It takes time and patience for them to settle down and trust us.
Make sure you place food by his hiding place so that he can get it without coming out.
Is he a single piggy ?
Guinea pigs are not wired to live alone but with at least 1 companion.
Ensure that part of the cage is covered to give a feeling of safety.
Spend time just quietly sitting with him beside the cage, talk gently to him so he gets used to your voice.
When I’ve had new piggies I’ve discussed current affairs with them or read them stories.
It doesn’t matter what the words are, it’s the tone of voice.
Looking forward to getting to know you