Renvy, this is difficult, because of course, we are not doctors and cannot diagnose or, certainly, treat anything simply via the internet. It is very important to consult a doctor who can tell you exactly what type of scleroderma your son has. It is most likely morphea, simply because that is most common in children, but it is worrisome that you say the skin over his whole body was thickening, which would more likely be systemic.
We do not recommend any severe diet restrictions (such as eating only vegetables) for anyone with scleroderma, and particularly not for a young, growing child, because regardless of type of scleroderma, he will need a full selection of nutrients for his immune system to function properly. I hope that you can find some medical advice and follow it. Even if you can only find a local rheumatologist to help, or find the biopsy results, that would be a step forward.
You may be able to figure out what type he most likely has, and thus what doctor to see, by looking at the videos on our main site about the various types of scleroderma. For example, morphea occurs in colored spots; linear causes an indented line of skin; and with systemic there is no color change, but simply a hardening that occurs on both sides of the body at once, usually starting in the fingertips and going upwards from there.
If he has colored spots of hardened skin, then it is likely morphea; and that is the kind that UVA1 phototherapy is particularly good to treat. See Scleroderma Videos now.
Please remember, we are not doctors. This information is only to help you find the best local doctor to help care for your son.