First things first -- you're a mom... and because you care, there are times you are GOING to overreact. :) IT's normal, healthy and allows your children to see that you're human too. Something that they can honestly lose sight of since they think we are the know all / be all people in their universes.
As far as a fifth grader playing with a kindergardener -- just because children are in a certain grade doesn't mean they're not sociable enough to play with younger and/or older children. We have a diverse age range on our street and they all generally play great together. I honestly wouldnt worry about that at all. I would do as a prior post said and just drop a note to the mom to make sure if he ever DOES go over there that she at leasts asks him if he has asked his mom if he can be there...some kind of secondary safety net.
Kids are kids and they will forget things. So keep that in mind. I'm sure he didn't do anything with the intent of deceiving you but at that age, they're quite narcissistic in nature and the world really does revolve around them and what is fun at the moment... (though that actually may last through adulthood for boys! ha! -- I can say that -- I have two! LOL)
As far as yelling at him in front of his friends -- me, personally, I would take him aside and apologize to him. Just let him know that you know that you overreacted and that you are sorry but that you did it because you were so worried. Again, sometimes letting your children know that you are human too will help you connect on so many other levels. Especially if it's not a normal thing for yelling to occur at your home.
Do be careful with the phone too -- I have a 7 year old and he is just an average kid -- things will get lost.... regardless of how much importance you place on it because it's more of a commodity to you than it is for him.
:) Good luck.... it's a hard thing being a mom sometimes! :)