Oil paint I think..but I'll double check. He's going to do a little more work on the helmet camo before moving on to the rest.
The great thing is he loves doing it
"I am using Vallejo acrylics and thinned with distilled water over Tamiya gray surface primer from a rattle can. Brush painting with very thin coats allows for easier blending and very quick dry times. After the primer was dry, it took about 40 minutes for me to paint the face on the head with the camouflage cover. Mixing neighboring colors as a thin wash helps to blend adjacent areas to make transitions less stark. Thin, opaque coats also leaves the details looking sharp. Also of GREAT importance is the fact that this figure is extremely well sculpted!"