Help! Settling another dog in

I've just got another dog (since 6pm this evening!) to keep Loki (11 months) company.
He's 3 months and entire (Loki is castrated). He's an absolute sweetheart except when he and Loki play Charlie (the new dog) soon starts to hump Loki quite aggressively - until Loki is in a tummy-in-the-air-submissive position.
Loki still wants to play but is obviosuly cheesed off with this.
They are both highly excited at the moment and are in separate parts of the house.
Any advice on getting them to calm down around each other and stop the humping would be great.

I think it`s very early days yet. Maybe putting a house line on the new boy and calmly separating them when the humping starts would help? Then praise him.
It takes a long time for a dog to settle. He`s probably just asserting himself because he`s anxious.

I've had a development. The obsessive humping turned to quite aggressive humping and he simply would not let Loki alone. On talking to the previous owner apparently he's not very well socialised and humps a lot -not just a stress situation.
We agreed that they would take the dog back before all concerned got too stressed. He either needs to live alone or with a more experienced dog and owner!

Feel really bad about it but it's the right thing for both dogs.
(bizarrely got rang by someone that I was in contact with about an failed drug search dog who has come up for grabs today, so all might not be lost.)

the thing is though, will the new potential dog and your own dog be able to meet one or two times without commitment to make some sort of assessment?
otherwise, you just risk the whole thing happening again.
also, you would need to be way more thorough in garnering background information about the new dog in advance.

and if that all works out, then on the way home you, and ideally a second person, should take them both for a big long on-lead walk before going thru your own door.