I though I'd take a cute kids picture and Iron-on transfer (Walmart Crafts) it to a T-shirt. "Girls Love Me" type thing. They are 9 and 4.
Maybe you could dress your boy with a tie and grown up stuff and say "To the Big Guy from the Little Guy" or Love from Your Little Man. Take a digital photo and print on Transfer and Iron On. If you are not graphically inclined the type can be printed on a separate piece. I've made all kinds of Stuff for our Daddy but he likes this kind of thing best. Or if you want a hands-on craft maybe your son could make a Best Dad trophy. I also had one make a handprint mat/frame and took a picture of her (Painting naked except diaper at 1yr) for the picture.
Mom of 2 great girls who LOVE crafts and Messes