Traumatised is an exaggeration of course but I didn't expect him to get teary and scared about using it. I cannot imagine how girls of that age cope with periods. (I don't remember how I coped myself but I was 12.5). I even offered for him to put the deodorant on me first. In the end I had it all up my arms before we got it under his arms 
DS has started to smell in the hot weather so I thought a simple deodorant would be a good idea. He is 10.11 and year 6.
Any other parenting tips welcome for these young developers welcome!