My DH died just under 4 months ago, aged 46, and I have some insight into what your partner's thought process might be. There are a number of things and it's fairly complex and I think you have to be fully comprehending of this stuff to get where he's at in his head.
Firstly, the fact he is deeply grieving his late wife does not mean he doesn't love you as much. You and his late wife are two different people, but what it probably means is that they had a good relationship and because of that he craves the same again. If they had had a terrible marriage perhaps he wouldn't be so cut up about losing her or so quick to be with someone new. It's natural that he is grieving not just the loss of her, but also the whole 'being in a relationship' way of life, and I think that's why he has moved on so quickly with you. Probably incredibly lonely after losing his wife, missing the companionship, missing having someone to 'do life with', being part of a team, being best friends, probably suffering with 'skin hunger' (it's a thing, I currently have it badly - missing being touched, missing sex, missing physical intimacy, cuddles etc). Having all those things and then suddenly losing that person is incredibly traumatic. He will also be thinking about everything she has lost, and that in itself is a whole other pain that mingles in with the rest.
He is never going to stop grieving his first wife, he loved her and visa versa, and they didn't split up, she died. The end of their life together was not a choice, it was forced upon them. He will always miss her and think about her, so that's something you have got to be okay with because it's natural and would be a disservice to her if he didn't. If you suddenly died he would be the same about you I'm sure - and I'm sure you wouldn't want him to forget you or put all his photos of you in a cupboard because somebody new came on the scene, that would be horrible, and cold. He needs to be able to grieve and that could be life long - HOWEVER, it absolutely and categorically does NOT mean that he loves you less. I have no doubt he loves you very much. Those two things are co-existing inside him. For myself, I do believe I have the capacity to love somebody new one day (although I have very complex feelings about that) but it won't change the fact that I love my late husband and always will forever. It would feel like a massive betrayal to put my love and memories of him aside to suit a new person in my life - what sort of person would that make me? And the same with your partner - surely you don't want to be with somebody who can just forget about someone they loved because they have died? You wouldn't want him to forget about you if the worst was to happen?
What I am getting at is that the fact he is still deep in grief is normal and shows he has depth and integrity. I would judge somebody who moved quickly on but then seemed to bury all thoughts of their previous spouse, that would be really cold and unfeeling.
I do think getting into a new relationship 6 months after the death of a spouse is too soon, but I can see why it would happen. Loneliness and the craving of that companionship and love and intimacy - it is really fucking awful to lose that, it's so so awful, beyond describing. So I think you need to give your partner the space and time to grieve and don't take it personally. You have to understand he will always carry that grief - but it absolutely does not mean he doesn't love you just as much. I think getting into a new relationship that quickly probably does complicate the grieving process because I don't think he's given himself enough time to process his grief before you came along, but I guess that is something you both now need to muddle through. I do think that getting involved with somebody who is widowed is quite a brave thing to do, and you have to understand what you are getting yourself into, because there are emotional complexities involved that there wouldn't normally be in a straight forward relationship. I actually think to myself, who would ever want me now, a woman who is still in love with her dead husband? There are photos of him everywhere, his stuff is all over the house. But I still have capacity for love, loads of it. I don't think people who have been widowed should be consigned to a life of loneliness forever because they once had a spouse that they loved. It just takes the right person with understanding to help them move forward in life.