if you've not already done so, I would recommend getting him checked out by your Doctor, just to make sure theres no underlying health cause,
but Raynauds can be a problem in its own right & it can also be a sign of dietary deficiency, upping his intake of Magnesium.....often a VERY commonly deficient mineral due to out modern western diets, & less of it in the soil than there used to be, ......this one is easy to deal with, ....add 2 cups of Epsom Salts to his bath & let him play in it for at least 20 minutes....it will soak in through his Skin.(proven medically to be a very efficient way of upping your bodies magnesium) ..if you want more info google "the "Epsom Salts Council"
Fish oils are also meant to be very good for easing symptoms, so if he's not eating a lot of good oily types of fish, such as Salmon, Sardines, Mackeral etc, you might want to look into a good fish oil supplement, theres lots aimed at kids these days
I get Raynauds & if bad, which his sounds like...it can be pretty painful, so I don't envy your poor DS, thankfully mine didn't start until I was 19, & I can vouch for the above being a big help