My DS6 chewed a lot of things too, and he hummed (still hums occasionally and the chewing is nearly absent). It used to drive me crazy- not so much germs, but worrying about all of the chemicals or metals that seem to be in so many products.
When he was little, the humming was something that we noticed right before a tantrum. In fact, it was a late warning system that he was nearing a breakdown.
I think it may be sensory (he complains that he can't concentrate with background noise- neither can I for that matter). The chewing tended to happen if he was overstimulated or frustrated- although he chewed his pencils when he was concentrating (I still do that:)
I made a big deal out of it, and I think it actually made the problem worse. He responded to my anxiety and he also discovered it was great way to push my buttons
The chewy things didn't work because it actually seemed to encourage more chewing, but parents whose children have severe chewing issues swear by them. Gum would be a trap for us because my DS likes to "game" the system and would chew just to get a piece of gum!
We do give crunchy food (whole apples are great, love the idea of beef jerky, we have even suggested that he chew some ice), and I don't know how well your son reads, but once mine reached the point where he could read for enjoyment, 10 minutes with a book calms him down. I've even explained to him that in addition to all of the great things about books, they focus the mind and calm the emotions.
