Ah...I see...
The trick to preventing hypos -like that one - is to be sure to get a 2+ . Lantus takes about two hours to reach max effectiveness...
It looks like he's rising now, you should go ahead with your regular dose. Please be sure to get a 1+ and a 2+ if you're nervous about which way he's heading.
The #1 defense against hypos is a good offense! Test, test, test...and you'll see those numbers starting to drop...anytime there's a bigger drop than you're used to, a teaspoon or two of MC, and watching him closely will help stave off those lower scary numbers.