Dd2 is identikit. She couldn't really feel her face/chin so had no idea whether she was wet, or even if she had food all over her face.
We tried hyoscine patches (great - and her speech clarity improved hugely as she had less slobber around) but then she developed a skin irritation and couldn't tolerate the patches, they left big red weals on her skin.
Tried oral medication of the same - again, brill. But when she was on the tablets she had nose bleeds, which stopped when we took her off them, and started again in we put her back on them...
We were also offered Botox on the salivary glands.
- and ultimately surgery to reverse the salivary glands.
(neither of which we took up)
So dd2 went to school wearing bandanas and sweat bands on her wrists to encourage self mopping (as a habit - she couldn't feel it when she needed to), and then eventually we ditched bandanas and just sent in dry sweatshirts.
She continued to improve, and at 8 it is only really associated with intense concentration now. Her tongue motility and sensation around her mouth is also improved.
We've done face tapping over time, and gazillions of SLT exercises (all the usual candles, licking blah, as well as mouth aerobics etc)
I like the site www.new-vis.com for feeding and associated stuff - there used to be some stuff on there about stimulating sensation etc, primarily intended to promote suck for tube fed children, but a lot of it I found really useful for dd2.
Dd2's sensation is still not 100%, but last month she finally learned how to move her tongue laterally in one direction (she's 8 and still can't move it the other way lol), so I think you just have to keep plugging away at al, the exercises. Development still keeps inching along. 
(I bet he over stuffs too? Blardy bananas are the worst. If you can't feel how much you've got in your mouth, it's really easy to practically choke yourself, or be too full to chew!)