My Son Who Is Just 7 Has Had Recurring Stomache

My son who is just 7 has had recurring stomache aches on and off since Xmas. He tells me about it 3 or 4 times a week but says it happens each day. His stomach is often hot to touch when it happens and he sometimes has a slight temperature. He has also been very pale and had dark circles under his eyes since Xmas. Five weeks ago the temperature was severe as was the stomache pain and I think he had a virus ( other children at school did too). Then 3 weeks ago he had gastro enteritus. Also on a couple of occasions he has woken up in the morning with severe throbbing headaches. When he has the stomache ache he has severe cramps and often cries but within about 45 mins he is ok. He is most likely to get the pain after eating. He has been to the doctors twice and they have checked him out. They think on top of the viruses he has been sufferening from abdominal migraines or impacted poo. This may well be the case. I have followed the instructions of the doc to fill him full of fruit and veg and raisins and water and try to run his poo loose. Despite my best efforts his poo is not loose and he is showing no signs of improving. This is a child who was previously well and only had about 1 day a year off school and had never had stomache aches. I am quite worried about him and just want him to be well again. he has had poo and urine samples which came back negative. Should I go back to the docs? Could it be a dairy allergy ? What do you think?