does it take for your body to start adjusting and make improvements?

My son has ben diagnosed with ADD and I've finally convinced him to go on a highly alkaline nutritional diet - Fruits, vegetables, some raw nuts, and small qty of lean meats + filtered water and only olive oil and basalmic vinegar for salad dressing. He's been on the SAD diet for 28 years - big time. I've asked him to stay away from simple sugars, processed foods and meats, most all condiments, caffeine, alcohol, saturated fats, msg, and artificial sweetners. I've also asked him to take a good multivitamin (allergy free) and some omega 3 and 6 supplements.

He's been on it for just two days....but in the short run is feeling worse in terms of his ability to focus and remember facts. I've told him that his body is going thru a withdrawal period from all the junk food and he needs to just give it some time.

I don't expect this diet to perform miracles but I would be interested to know other peoples experiences when you leave the SAD diet for a more nutritional 70/30 alkaline balance diet. Can anyone tell me what to expect in terms of time it might take for the body to resond to a proper diet when you leave the SAD diet?