Hi Anne. This was one of the best fitting masks for my 5-year old son. He is small for his age and is now 7 but pretty much the same size. It still fits great. We did have to look around and try a few masks to find one that would seal and fit a small (child) face. The best thing would be to have him try it out and see if it fits and is comfortable. It was for my son when he was 5 and about 40-45 pounds. It seems to adapt to a variety of faces. Just make sure it stays his mask, so it doesn't distort to fit an adult or older child's face, as then it may not fit as well over time. It certainly works well for a thin adult / adolescent face too, but the small volume seems perfect for younger children. My son has used it for fun in the pool as well as snorkeling at warm-water caribbean beaches. The perfect accompaniment is a small dry snorkel where the water doesn't get in when you go underwater. That made a huge difference in our son's comfortableness with using the mask not just for holding his breath and going under water but for snorkeling in the pool and on the beach, since he no longer had to worry about water splashing in and getting in his mouth or blowing out the water. It just instantly seals water out so it keeps the snorkel dry. You have to pay attention to the dry snorkel sizes for young children too as most seem to be developed for adult mouth and teeth sizes. We found a nice one (Oceanic Ultra Dry Mini Snorkel) that worked well for our son and his smaller mouth. It was much more comfortable for him than the regular dry snorkel. If your son isn't yet ready for snorkeling, but just wants the mask for fun and to see clearly under water (which definitely lead to increased swim skills and confidence in my son) while holding his breath, then you probably don't need to worry about adding a dry snorkel yet. We also added one of those neoprene thingee's to the back of the strap so it doesn't tangle or pull in his hair, but it's fine without it. Hope that helps!
Thank you so much! Your reply helped so much! It's been so hard to fugure out what size to get him and now I know that this one will probably work out great. He uses the mask to swim in pools while we are traveling and he feels more confident with a scuba mask on when he's swimming. Again, thanks for your help.