Can I suggest not using a potty chair at all! For my son it was much easier to get him used to a potty seat that fit on the big toilet, that way you don't have to do another transition back to a big toilet from the potty chair. Plus if he is interested this young it will maybe stave off any fear of the toilet (neither of mine had this). Not to mention they are much easier to clean then a potty chair.
You can get them at Target for like $12 , all sorts of fun characters. Also One Step Ahead has a portable one that goes with you in your purse when he is further along and goes in public.
Yes 20 mos is very young it is great he is interested though!! Boys are a whole different beast in the potty training.
Let him have fun with being interested, encourage him but do not push anything so it doesn't backfire on you. Him being interested is a very small piece of the puzzle. He has to be able to physically hold it, wake up dry, know the urge, pull pants up and down and so on.
I got both of my kids potty seats for each toilet in our house, then a fun step stool (that step stool I can say was the best purchase we have bought for brushing teeth, washing hands and all).