First, if he is not interested all by himself, please wait a year.
Second, if he really wants to print letters, it would help if we knew more.
If he is having difficulty holding the pencil correctly, then there are lots of fun activities to build the grasp (lite brite, pick up sticks, playdoh and cutter, etc.)
If he is having making individual letters, is it a copying difficulty? Make suares of paper, each with one symbol in it ( a horizontal line, a vertical line, an angle, a cross). If he can copy these, flawlessly, his brain has incorporated these movements. If he cannot, it is to early for him, and he should be coloring, drawing, painting, cutting and gluing, until his developing brain naturally matures. Beginning too early causes poor writing posture and a hooked hand. Very difficult to correct later.
Also, please know that capital letters can be very difficult because so many have angles. Often children do lowercase letters easier.
Best of luck to you both.