Kitten runs away when he's not in the basecamp

Hi everyone, I adopted a 3 month old kitten and I've had him for about 8 days.

I set up a room in my house as his base camp :)

In these 8 days, he's made great progress. The first few days he wouldn’t come out from under the bed, but now we actively play together, and today he purred and let me pet him for an hour.

All of this happened in his base camp.

My concern is that he has already explored every room in the house, but as soon as he sees me walking, he runs away and goes back to his base camp. Outside of his room, I can’t approach him.

During playtime, I tried slowly encouraging him to play outside the room, but when he reaches the door, he freezes and runs back under the bed.

Another thing I’ve noticed is that he isn’t afraid of me when I’m sitting or lying on the bed with him, but as soon as I stand up, he suddenly runs away :/

Ideas to try to relax him with my presence?