I also have a 20 months old son, and no, I haven't bought him any standard LEGO sets yet. I did gave the two buckets of used Duplo bricks that I had, and bought him two more buckets, but I'm waiting to see if he really does have an interest in LEGO before I go all out.
If he does like LEGO, then we'll have plenty of fun together (even if we have to use my own collection). If he doesn't, well, I'll encourage him in any way I can. He's the most important thing in my life right now, so I will be interested in whatever he will like later on.
But, from what I've seen, usually, if you're passionate about something, there's a good chance that your kids will at least be interested in what you're passionate about (well, until the ungreatful years of adolescence, that is). So far, I love hockey, and my son seems to like hockey too. So there's hope I guess!