The letter of the week idea is a good one. We actually had a Sunday School curriculum for preschoolers with that theme, and we did the uppercase letter one week and the lowercase letter the next. That way, our curriculum lasted for an entire year.
He’ll get used to the idea of upper and lowercase letters as he sees more and more printed material and learns to read words. As you are reading to him, you can point out, “Oh, there’s our friend, Uppercase A–and there’s his little brother, Lowercase a–both doing their important jobs of helping us read!”
One thing we did that was a lot of fun was the “Mystery Letter Box.” I took an empty shoebox–a big one, from boots or men’s shoes–and I covered/decorated the box and lid. Just make sure you can still open the lid. I cut a three-sided door panel to bend inward into one of the short sides of the box bottom. Then, every week (or 2-3 times per week) you put a few things that begin with the sound of the letter of the week in the box. Put the lid back on and have your son reach into the box from the side door to see if he can guess what is in there. Remind him of the beginning sound of the letter of the week. When he guesses, either he can pull it out of the door (if it’s small enough) or you can lift the lid to take it out without exposing the other items.
Conversely, you could have him go around the house looking for appropriate things to put into the box to show dad later. Or how about a Letter of the Week Scavenger Hunt, where you place the appropriate things with the letter attached around the house or yard, and he has to find them.