When I was teaching (kindergarten), it went both ways. Sometimes I got the usual box card as the students got, and sometimes a student would get me one of those teddy bears holding the box of chocolates, or a little something "extra".
However, since your child has only been going there 2 weeks, I don't know what to tell you. Since he is not there full time, how often does he go? You can certainly ask if there is going to be a party of any sort, or ask if the other children will be exchanging valentines.
If it were my son in this situation, I would ask if the children were exchanging valentines. If they were, I would include something for the teacher - maybe a nicer card and have my son pick out a candy bar or something. Or maybe look online for cute sayings to include with a little fruit basket - I have seen cute sayings for tags on a banana or orange (healthier options). If it was a preschool teacher that I had a real relationship/connection with, or if my son had been going there a while, I might get a small gift card ($5-10) for a local coffee shop/ice cream shop.
If you did get a "gift" I wouldn't spend much. Just a little something that shows your appreciation. BUT, again, don't feel obligated.