Some of that is very common for a boy this age. There is a researcher who did great work on the ways boys and girls communicate. Girls (generally) communicate by trying to find common ground, i.e. show they are the same, even mimicking speech patterns when responding. Boys (generally) communicate by trying to "one up" each other in a sense (she uses the term "negotiate their status within the group"), even when it leads to tall tales that the other children clearly know are not true/impossible.
I work in early childhood and I find this to be a great argument. Individual children can be exceptions of course. Also, this makes the boys sound terrible but if you watch the video and see her video example of the little boys talking about how high their ball can go, it's actually an adorable and charming conversation where the boys are more delighted by the thoughts that their friends are coming up with. It's fun for them, a game. The conversation between the little girls is similarly adorable and charming.
Here's the video with examples:
http://youtu.be/0XO0XJHesUU
So I don't let the kids push other kids aside to get praise, etc. If they're in a conversation laughing about how fast they are, then I won't say "now, now, you know that isn't true." It's not a problem. But in the case of the picture, I WOULD redirect. I would say "I like everyone's picture, but I was telling Johnny about how nicely he used those colors. When Johnny and I are done, you can tell me about your drawing." They have to learn appropriate social cues.