A lot of rabbits seem quiet and lonely on their first day at a new place, its natural, theyre out of their own territory so feel a bit unsure. In a few days he'll be settled in.
Personally, nowadays i recommened always getting a bun a friend. I had two females years ago that were meant to be bonded, but it didnt work, and i thought they were fine so didnt try getting them a new friend. However, one of the females has started getting on with my new female and male, and im going to bond them properly, as she seems SO much happier and content, cuddling up and grooming.
So in my opinion, i would buy a female. That doesnt neseccerily mean go and buy one tomorrow however, if you dont have the money and space for another age to keep them seperate until theyve both been neutroed, waiting until 6 weeks after his castration would be fine, and you could rescue an unwated female from a rescue, there seem to be many female buns around 1 in rescues, which would be perfect as the age difference isnt too much! Good luck.