this is more likely to be related to his diabetes than it is a std. unless you suspect him of having other partners, no reason for std testing. if he hasn't had a prostate exam recently, might be worthwhile to get one done too. If he keeps urine dipsticks around the house for his diabetes, he should try dipping his urine when it burns to see if he has glucose or ketones in his urine too when it occurs.
grace
I missed that he was diabetic;
Post in Urology community.
We never really know. Based on this one symptom its hard to say.
If you concerned it an STD: have him test for gonorrhea and chlamydia, and for urethritis.