Hello!
I started when my baby was your son's age. I would take two different cards, tell her: this one is a dog (flip it over) and this one is a tree (flip it over), then ask her where the dog was. I noticed she would be picking them following the same pattern (only the cards on the right, or only the cards on top) Wasn't very successful overall.
These days, I lay down 6 cards (see the attached document), give her one and ask her to find it and to pu it on top of the corresponding card. The first time she would point at any card without even looking (
) I tried again that same day and she very happily matched them all.
I try not to do this activity too often though, as I think they lose interest.
With the document I attached, you can try with the first set, then the second set a few days later, and finally the last set.
If you try on this bear matching game, let me know if your son tends to mismatch the ovals and the circles like my daughter :-)