Sounds like you have an adorable little guy there! I can relate to that "jokester" personality and I am sure he can speak a lot more than he is letting on. Seems to be one of those little kids who don't talk and then one day speak in full sentences.
DS2.5 has a similar personality except he is very verbal. He messes with me too though...When we read he will point to 2 words and say what they are but switch them up! He thinks it is hilarious and cracks himself up...lol! If I ever ask him a color, number etc., most of the time he refuses to answer me OR he jokes around and says the wrong thing all the while laughing hysterically.
This is very different from DS4.5 PG. He was always very serious and would never think to joke around about "serious" things like that...:-)
Pretty sure you have a gifted child there but wait with testing. We got DS4.5 early entrance to K this fall just by showing his work. We since had him tested and especially the achievement test will come in real handy once he starts K. There will for sure me some subject acceleration needed.
I suggest you DO have him tested eventually just because it hold so much more weight to present a professional evaluation rather than the school just taking the parents "word" for it..
Enjoy the ride!