What country are you in? In many countries when a person get diagnosed and starts treatment, there are follow up tests more frequently than you have reported for your friend. 8 months?
I was tested every 4 months my first year. on treatment.
So anyway, sure, when he gets the undetectable result, that will calm your worry a bit.
Keep this in mind. I had HIV+ boyfriends when I was HIV- and 2 were before there was treatment and HIV infection meant death. I used condoms and continued in the relationships without getting infected, but its not like the worry could every evaporate 100%. I think I had more "will" when I was young.
All you are going to get is the science proof that if your partner is undetectable and you use condoms, that is 2, count em 2, reassurances that you are not going to get HIV from him. So try it with that reassurance. If you STILL have too much anxiety, then you have to cross that bridge - figure out how to manage your anxiety, or break up.