E.J.
I want to applaud you for noticing that he may be hard to understand. My friend has a boy who at 5 I couldn't understand a darn word he said and she was in denial and refused to get him speech therapy.
I think an evaluation wouldn't hurt. The other Mamas have already given you great tips on checking with insurance, etc. But I think to take the time to have him evaluated will give you a lot of answers.
My son started speech therapy at about 12 months old. (I think... hard to remember). Anyway, he didn't speak until he was 3 and we signed before that. Speech therapy was wonderful b/c not only did she work w/him, but she gave me a lot of great tips on how to enhance his speech at home. Now he's 6 (will be 7 in June) and he never shuts up!!!! I'm so thankful he got that intervention that got him back up to where he should be. You'd never know now that he ever had a problem.
Good luck!!!