I think you have already had some good advice. I was a policeman until invalided out with the usual suspects, PTSD, bouts of depressions and ongoing anxiety. I'm afraid I'm not sure of the wisdom of anyone with an anxiety conditon being a member of a force, however as he is now in the best strategy is to take that into account.
The first thing I'd have to say is that PTSD is not a given, some people can handle the more traumatic side of police work than others and basically get though their career unharmed. The second is that your BF needs to take an intelligent attitude to his career, and not soldier on if things are starting to affect him, but shift to another area. Actually regularly shifting areas is probably a good idea for its own sake and for one's career.
Such a career can be interesting, challenging and not necessarily harmful. On the other hand it can get to seem 'the only think in the world' (my mistake).
As time goes on there is a danger one can become blinkered and make police all of one's life, both on duty and off, even socializing almost exclusively with colleagues and their families. This is not necessarily good. While there is no doubt police work is highly demanding and forms probably the majority of one's life and social connections with workmates are necessary there does need to be an attempt at balance, with interests completely divorced from work.
If your BF ends up with an experienced unofficial mentor who can tell him if he is getting too close to his work and liable to suffer mental health effects from it so much the better, sadly I had no such person.
I guess for you the best support is to keep an eye open for adverse reactions and speak up at that stage - in a firm but loving way. Encouraging him to seek professional help early if it is needed. There is no need to know the details of every traumatic incident he faces, that can have an adverse effect on you, something you don't need.
There is no room here to go into all the warning signs one might encounter, however the more distant he appears, and of course any very marked change from his normal self, should be noted. Friendly discussion with him early on is good.
You are welcome here at any time to discuss whatever concerns you, it is not always easy being a policeman's partner.