Creek was 6 when we added little Caney to the mix. BF thought that Creek would enjoy having another dog around, but honestly I was not too sure about that before we brought Caney home. Creek has always been(and still pretty much is) an independent spirit; she likes to do her own thing and always finds ways to entertain herself, and she seems to prefer it that way too. Creek LOVES people but from what I had seen she really could care less about other dogs. But we wanted a puppy, the time was right, so we got Caney.
I won't lie, there was definitely a HUGE adjustment period for Creek... she acted jealous, she pouted, she sulked, and she wanted nothing to do with the puppy. Little by little she did warm up to her though, but it was VERY slow, and I swear they STILL grow to like each other a little bit more with every day. They do act like "sisters" in that Caney still spends a good bit of time pestering Creek to play with her
but at the end of the day they really enjoy each others' company and find comfort in it. They have been apart several times for a week or so each and we were a bit surprised the first time that Creek even seemed to miss Caney. That pup has definitely grown on Creek!
Also, getting new puppy was JUST what Creek needed to get her up and going again... she was beginning to "slow down" a little but that changed almost immediately once we brought Caney home. Now she has someone to keep up with
I think that your dog will adjust to a new puppy in time, as long as you remember to give him plenty of love when the new pup arrives so that he knows he hasn't been forgotten. It might take a little bit of time but especially as the pup gets older I think your guy will come to appreciate the companionship. If Creek can adjust to having a "sibling" then ANY dog can