I got my schnauzer cross Jess when she was 3 years old (she is a rescue) and she is now 13 years old.
I got Poppy at 6 weeks old and she is now 6.5 months old.
Jess has always been the only dog and is totally spoilt so she was a bit put out when we first brought Poppy home.
At first she wanted nothing to do with her and would keep her distance, then she started to put Poppy in her place and let her know her place in the pack was FIRMLY below Jess's
They are now good friends and even though sometimes Jess cannot be bothered to play with her she does tolerate her. She seems to enjoy the company.
I think as your dog is still young this would be a good time to introduce a new dog if that is what you want to do.
You can expect him not to be happy about it at first but this happens with every dog and he will eventually warm to the new dog.