19 months is still very young, especially for a boy. My son started showing interest at 18 months, but at 27 months he's still in diapers. IF he's showing all the signs that he's ready (staying dry for long periods of time, telling you he needs to go, is able to sit on potty for a couple of minutes, had regular "predictable" bm's, shows interest in your bathroom habits) then you can probably start now. Until he's going on a regular basis on the toilet, he needs to start sitting down. Otherwise there's control issues and you'll just have a mess to clean up!
Don't rush him, though, just b/c you're tired of diaper duty, otherwise it will just make him have setbacks. Follow his cues and don't push him. If you start, and he's just not interested, then wait a month or 2 and try again. I know some moms swear by the "potty train your child in 3 days" method, but I think that only works when the child is truly ready. Hopefully you're one of the lucky ones whose kid is ready early! I'm not trying to be all negative about this, but I got excited when my son showed interest at 18 mo., then a little disappointed when it didn't work out so well.
I went to www.babycenter.com and looked under the toddler section and looked up potty training readiness check list to get just a few of the signs. It's a great resource (for lots of other things, too!).