Went thru that 35 yrs. ago. At the time they ground up the whole bee, now they just use the venom. How they get the venom is anybody's guess.
If his reactions at this time are not life threatening, I wouldn't do it. Here's what I know.
At the time I started into the program, they had me read the scientific literature. Thinking on some things may have changed, but here's what I learned: Desensitization can work, but it is not without risks. You can get a life threatening reaction from the shot, so shots should NOT be given at home (although one doc let me do it, it was foolish of him & me). Each time a person gets stung, he/she can become more allergic. Desensitization consists of shots of venom in very gradually increasing concentrations BUT if the program is stopped the person may end up much more allergic than before, because the shots represented more stings.
My shots were discontinued after several years for various reasons. With new health insurance, I asked to be retested. The nurse said she had never seen anyone react so quickly, so my sensitivity did get worse because I was treated and later stopped the treatment.
So, if you go for the shots, they probably need to be continued for life. 3 yrs. works for other hay fever, but I would quesiton the wisdom of stopping treatment for bee stings. And with the hay fever shots, the effectiveness diminishes over time & they have to be resumed again after a few years. Same ought to happen for bee sting shots, but as I said, with bee stings if you get worse, it could kill you. Question the doc on this. Is he an allergist or a GP?
In my own case, we don't have a lot of honeybees in our area. The doc & I mutually decided that I should just carry a kit because the probability of a reaction from a shot given every month or so is greater than a probability of a bee sting.
Also, be aware that yellowjackets, wasps, hornets & bees are different. You can be allergic to one but not another. We do have yellow jackets around here & I react to their stings like anyone else, but a bee could kill me.