I am not qualified to offer my own opinion, but I think discussing it with your vet would be a good idea. If you do, make sure to let her know you are worried about the toxicity of vitA and giving Herbie too much. Also, from the reading I have done you probably would want to hold off on giving preformed vitA for a while after giving the shot.
If Herbie's food source contains beta carotene, he should be creating the vitA he needs. There can be other factors though that interfere with that process, and that is where my understanding begins to break down. Wish I could offer more help.