20 months is not too young to begin serious potty training. I would probably have began him much sooner. When they can walk well and tell you they want a food item I would consider that the time to go into training mode. All three of my children were potty trained by 2.5 years old. But, I must admit that I was fortunate that their bowel movements became pretty predictable. As far as when they urinate... I found that using training pants... the cloth kind that you use with a vinyl cover. They don't want to be wet and the pull-ups take the moisture away from their skin so the event goes by almost unnoticed. I would only use the pull-ups when you leave the house. Another thing I notice about children is that they understand a lot verbally long before they can actually communicate it to you. So when you repeat a behavior many times.. they get it. When the cloth training pant are wet.. you say..." you pee pee?" have them touch it.. and repeat " you pee pee".. Take them to the toilet and point tell them " you pee pee in the toilet understand?".. the first time( and 2nd 3rd 4th) they may look at you like "what?" but after about a week or two they start to get it.. then you just reinforce and stay consistent. You can also have them sit on the potty while your husband uses the big toilet and say" daddy pee pee". at around three or four years old you can correct what they call it.. but at 2ish I think it's fine to say "pee pee"
Whew... I know you probably didn't want all that but.. there it is =). I hope this helps and I would love to hear some updates on the little mans progress. One more thing. .. it absolutely depends on each child since each of them is different and learns in his/her own pace and own way. Eventually we all learn to use it =). I don't think it matters too much when your an adult. I'm sure you don't get a job based on when you got potty trained. It's more of a convenience to the care giver. It's also lighter on the wallet( and nose) to have them use the toilet. But beware.. they will have to use the bathroom in public eventually in the potty training saga... and that truly sux as you probably know from your daughter... Anyhow.. Good luck!

Thanks for the tips, mcclainmom! One issue I've run into is that the little guy isn't much of a talker, so he's not very good at communicating when he needs to go potty. Pretty much I'm the only one who understands his signals, so it was turning into a lot of pressure for me to be paying attention to him ALL the time (and then any childcare situation (e.g., church care) was just not equipped to help in this process, 'cause he's so young). In addition, grandparents have been visiting and they have been kind of distracting. So, I think I'm going to continue letting him use the potty several times a day, but without the pressure of "training" him at this point - I'm going to wait a few weeks and see if his verbal skills improve enough that he can more easily tell us when he needs to go (he's going through a big spurt in verbal delevopment right now, so maybe he'll be there soon enough).
The other thing I wonder about is - from when I start seriously "training" - how long should accidents continue to happen? I think I want to give up too quickly, but maybe I shouldn't expect this to happen so quickly. I don't know whether the accidents are just a normal part of the training, or if they're a sign that the timing isn't right.