12.5 yo Lab chow mix recently lost his companion who was 14. Not accepting new puppy?

Otis is our man man and he knows it! We recently lost our Abby who was 14 and they had been together since he was 8 weeks they were very close:( Little background on Otis he is a very sweet healthy for his age and very fit and active. He was well socialized when he was a puppy however he was attacked several times and has since become the aggressor. (He actually tore apart a groundhog in a matter of seconds) So over the past several years we have kept him away from other dogs. On Thursday night we brought home a 14 week old golden doodle and we have been trying to introduce them she perks right up and wants to go to him but he does not want anything to do with her he literally will not look at her and runs the other way when she comes near him he has not growled or nipped at her. He seems to be good with us as well still wagging his tail and getting all his attention. We took them for a walk yesterday and he was good but if she charges at him he just runs away we also had them in the yard with her on a leash and him off so that he could just stay away or get away if she went near him. Here is my main concern I know they need to establish boundaries and I have kind of been putting myself in between them so she does not charge into him and have him bite her. She is submissive and holds back some but then she just can't take it anymore and runs over to him, he really does not like this. I would hate to put a mussel on him but wonder if I did then I could kind of see how he was going to react to her without our inter-fearing I just do not want him to harm her! A nip would be fine if I thought for sure that was all he was going to do but we have seen him fight and it is vicious! Would a dog training class be helpful to him as well to help him become more comfortable with other dogs? Or will this just add to his stress? I love him so much and just want him to be happy and the loss of our other dog has been hard on him and the whole family we thought the puppy would help but now I fear it could make it worse:(