On that particular jawbone, there is very little wear on the 3rd premolar, slight wear on M1. The correct textbook answer is 2.5.
I will say this, however...
This is a great example of why you have to learn the wear pattern on your local property based on prior years trail cam pics... if you had pics of him last year as a 2.5 y/o, then again this year of the same buck and you thought he was 3.5, then he most certainly is 3.5. If over the course of several years you see this same wear pattern, then you will be better able to age your local deer. ie, slight wear on M1 may be indicative of a typical 3.5y/o in your local herd.
Honestly, on my farms, that would be the case. After killing numerous 3.5 and 4.5 y/o's that we've been able to trace back to 2.5 through trail cam pics, I've found slightly less tooth wear than the 'textbook' age.