Bless him....he's just missing his littermates!
He'll be feeling a bit lost without them, just give him plenty of cuddles and somewhere he feels safe to run off to in case things get a little too much for the little guy......maybe somewhere upstairs thats nice and quiet.
We always advise new owners to isolate the kitten in a bedroom with food/water bowls and litter until the kitten feels it wants to adventure out of its own free will.....this way they realise the bedroom is safe ground almost straight away.
He just needs time to find his feet and figure out the pecking order.
Expect the madness to come out in him 7-14 days after he first arrives home!
We put special hand crocheted woolen blankets(one for each baby) in with our kittens beds about 2-3 weeks before they leave, they act as a kind of security blanket just to help them adjust easier to their new home.....maybe an idea to help your second kitty feel more at ease.