Nope. Go for it.
Does he already know--the new guy, that is--that you selected your original groomsmen months ago? If not, don't even bother dragging up all the details. He's in, and that's that.
If it's clear that he's a second-stringer, so to speak, tell him that you originally wanted him to be a groomsman, but you simply didn't have enough slots. Technically this is true. He has no reason to feel slighted or grumpy. He'll be honored. (If he's not, he's kinda being a dick.)
SEE ALSO: Can Your Best Man Do All This?
On a more general note, and at the risk of making a gender stereotype (our first!), this is why it's good to be a guy. When it comes to matter of decorum and formalities, at the end of the day... we just don't give a shit. Even if you did sorta slight your friend (you didn't), he might be initially miffed, but, well, he'll get over it. This won't cause a rift. He won't sulk and complain and lead a passive-aggressive insurrection: that's the job of the bridesmaids.
In general, when in doubt, trust your bonds of friendship. Trust that your good friends are your good friends. Friendships are long, weddings are short.