My boyfriend is 53. We have been dating 2 1/2 years. In the last 2 -3 months he does not want to have sex at all. Could this be because of a low testosterone level?

In a word 'yes'. However, there may be more to it than just low testosterone levels. Stress, medications, health concerns, exhaustion, etc. are just a few causes for a decreased libido.

A check-up can determine if there are any physical issues. Talking with him might find where life in general might be causing issues that he needs to work through on his own or that you might be able to work through as a couple.

Yep... what Cat1864 said...