I feel for you OP. I don’t want to give advice, just my own perspective in case it helps.
The first year will be very difficult for him, getting through every important date, all the family birthdays, Christmas, New Year, all the holiday periods, the build up to the first anniversary date can be just awful to live through, possibly a sense of relief after the day, but in many ways the second year can be worse, including the second anniversary. This time last year the family may remember all living through a difficult time of illness too. It will all weigh heavily.
Some other perspectives relating to telling other people/announcing new relationships:
I remember myself when I witnessed a widower form a new relationship, feeling awkward and actually judgemental (for no reason) when we were all (as bystanders in the community) still very much mourning his late wife. This is how some people will think/judge. Now I know better of course, that the poor man deserved every bit of company, happiness and friendship after losing his wife.
When I was widowed and met a new companion/confidant/live-apart travel partner - my closest and best friend was horrified - and openly stated that “she wouldn’t tell her own husband about my relationship” (because he would be upset!). I.e. their feelings were all about themselves being upset - rather than feeling happy that I had someone in my life to make me happier in difficult circumstances.
When my partner came to stay on his first visit, another friend suggested he stay in a spare room at her house (friend perceived it as somehow “wrong” for him to sleep in one of my rooms)
I openly witnessed a lot of conflicting emotions among my friends and the wider community who were mourning my husband - and this is what your chap might be living through as well.
People who haven’t lived through it, can’t be blamed for viewing someone new as “a replacement”, as something somehow disloyal, whereas the truth is that we should all be glad that the bereaved person is finding a way to make their life better, to have company, friendship, be less lonely and find a better life.