I would gather some alphabet books and start reading those with your son. If he likes letters he’ll love them but it’s not pushing anything on him, it’s just enjoying some books that he is interested in. I started letter of the week projects with my son at this age because after a week of some alphabet books he knew his letters. I had no intention of starting formally teaching him any letters until he was 2, but I followed his interests. If your son was into diggers you wouldn’t think twice about reading digger books. Just don’t drill him, playing with letters like other toys is perfect for this age, don’t worry about “right and wrong” either just model, praise and play.

There is different opinions about which to start first. When teaching to read/phonics, lowercase are vital to know, since they are reading and sounding out letters that are 95% lowercase. If you are starting letters early I say go for it any way you want. Ideally teaching both together. My son picked up his letters very early through alphabet books and has been fine doing Upper than lower which is the way most toddlers end up learning them. We did upper first because he picked them up, then started purposefully doing lower when he started showing interesti in phonics. Now that he is asking to spell we focus on lower more than upper.

I hope that helps!