You could use the foil, masking tape and papier mache as you planned.  I would cover the whole thing with a couple of full layers of papier mache and let it dry, then use an Xacto knife or box knife to carefully cut it in half, up from the bottom, across the top, down the opposite side.  IOW, from ear-side to ear-side.

I'm not exactly sure... are you planning to use the piece as a whole-head cover, or just part of it as a mask for his face?  Measure before you start, to make sure it will cover the area that needs to be covered.  If it needs to be lengthened, you need to know before you start.

So, if it's a whole-head mask, are you planning on having the entire bottom open so it just rests on his head?  That would be the easiest. 

After you cut it in half to get it off the mold, you will need to make the holes for the eyes.  An Xacto knife would do the best job.  Put small holes in first, fit it to his eyes, then gradually enlarge the holes, fitting it several times to get it right.

Then tape the inside with more masking tape to fit it back together, then add several more layers of papier mache so it feels sturdy enough.

The smoother you get each piece of paper laid down, the smoother the final result will be.  You'll probably still have to sand it a bit, but not as much as if you hurry with it.

You may want to put some soft elastic inside that holds the mask securely to his head, catching the elastic under his chin.  It will keep it on if he is bouncing around, and if the wind is blowing.

Sue