I've bought a fat tailed gecko around 4/5 days ago, he is lovely and was really calm at first, but now he's a bit edgy kinda thing.
I don't want to stress him, so i've avoided handling him, he's also not eating much, but i expected that while he get's used to his new surrounding's.

Does anyone have any advice on when the right time to handle him would be?

If i put my hand in, sometimes he will come have a nosey, but soon goes back to his hide

was he with other geckos from where u bought him? if so then he might be lonely. What r u trying to feed him? if it's boring to him he might not eat it. also u might wanna check your setup make sure it's not too hot or cold. I'm sure there r books or info online about what temps r needed if u r not sure. Hope this helps.

Lonely? I don't think the term honestly applies to reptiles, anthropomorphism springs to mind...