Yes, just oxidize the toggle and the bracelet should hold all the weight.

 (I haven't been here for awhile either...)
 

Sterling is just fine for chain, but as you alluded to in your post that AR is probably too big - I make roundmaille with 16awg 11/64 - your 4.5mm ID is larger than 11/64, and you're a whole wire gauge smaller.  You might have some problems, especially if he is going to wear it 24/7.  Now, the good thing about wearing it 24/7 is all that wear and banging around will harden the wire pretty quickly.  

If nothing else, I would solder the rings b/w the chain and clasp or double/triple those rings if you don't solder and check for loose or rings opening often - maybe once a week.
 

 The rings are stainless steel, and the rings aren't the problem...it's the actual pewter toggle that is wearing and getting distorted...wondering then if a sterling silver toggle would do okay...could oxidze it.

Why not just use a stainless steel toggle? You can get them at TRL, www.theringlord.org or maybe a local bead shop would have some in stock.

Just a note on the Lortone 3A running "hot."  I just bought one and the information included stated that the motor is designed to run hot and it is oiled for life.  Mine gets hot, too, even after two hours.