I have 4 kids, and only enrolled one of them in preschool. The other 3 have done just fine. the fact that he is the oldest in the group is good for him. My youngest is the youngest in her class -- and has been since kindergarten and while she passed her school readiness tests, I have been kicking myself for 8 years now that I didn't hold her back myself.
I would relax and watch the things he likes to do. He may just have a mind of his own, and he may like other things. But if he begins to withdraw and not respond to people and stuff like that, then pay quick attention. If it's just a matter of not being as verbal or paying attention to what others want him to do, it may be the difference between a boy and a girl. girls are more socially oriented, and will do what you want to please you, whereas my impression of little boys is that even at an early age they are more independent and driven by their own ideas. Sometime ask him to draw a person for you, praise him and keep it. 6 months from now, ask him to do it again, then compare the pix. don't be concerned with what's missing, cuz kids don't get this together until closer to being Kindergarten ready, but look for more detail than the first one had.
I had a child who held her pencil wrong no matter what I said or did. she was an A student.
the other thing you could do, if you want to, is buy magnetic letters for the refrig door, and spell his name so he sees it at home, too. but at 3, writing your name isn't a skill most kids have. Your daughter sounds like she had a real aptitude in this area, which makes it hard, then, to see if #2 is "average" because for your family, your average is already skewed by a precocious 1st child !
Good luck ! Parenting brings us questions like this all the time !! We try to chart the right course, and we sure wish there were more signs around to help us know we did it right !! :-)