This monster patch is perfect for him. He loves it! It was quick for me to fix (about 30 min) and I already had everything needed.
White felt or Fleece Interfacing
Pants with a hole
1 Cut out the red material into an oval about 3/4" larger than the hole on all sides.
2 Iron on the interfacing.
3 Cut teeth out of felt or fleece interfacing. If using felt sew along the top of the teeth. I used the interfacing and ironed it on.
4 Pin mouth to the inside of the whole and sew it. I used the machine and just went back and forth several times on the top and bottom to get a messy monster look. It was actually kinda hard because the pant lets were pretty small. Next time I might just hand sew it.
5 Cut out eyes and put them in place. I ironed on the interfacing but felt would be fine her too.
6 Either hand sew or use a machine to put a big messy X over the eyes.
I got the idea here. The blog is in Dutch.