Ds (14) is lactose intolerant. He was diagnosed with this when he was about 10, after he had a bad stomach bug wiped out his lactase enzymes, and since then, anything diary based and eggs have given him stomach upset. we cut dairy and eggs from his diet and he has been fine ever since.
Fast forward to recently, since around mid January he had been complaining of belly pains, started out a couple times a week, but past few weeks it’s been more regular, sometimes even daily. He has not been wanting to engage with the virtual school or homework (this is an issue from last year, home schooling has been a complete uphil struggle) and has just wanted to watch you tube, play Xbox walk the dog, and the moment school gets mentioned he says he can’t do it because he has belly ache. He is obviously going back to school next week, but this week has been saying he can’t go back because he is worried he will have belly ache at school and will keep having to go the loo all the time, so he said he is not going to go.
This morning, I had to pop out to the pharmacy, (was walking) DS was already out walking the dog - which he does every morning. I bumped into DS, and saw in his hand he was drinking a Frij (I think?) chocolate milkshake. He tried to hide it behind his back but I called him out on it, and asked him why he was drinking that when he knows milk makes him sick and he has been having stomach problems recently. He mumbled some rubbish about he just went to the shop and fancied this and that he “forgot” it had milk in it 
I came back and told DH and he has told me that he had also seen DS drinking that several times but DH had not realised it was milk. He said DS had told him it was non-dairy, but the description matched the Frij milkshake I saw him with this morning.
So basically, it seems that for the past six weeks of so, DS has been going out each morning with the dog, buying a milkshake and drinking it - knowing full well it will upset his stomach - and then complaining of having stomach aches - it’s like he is deliberately making himself sick!
I have no idea why he would be doing this and I’m thinking that this is like a form of self harm and that it might be a sign of a deeper issue? Or maybe I’m over thinking this? I don’t know how to approach him about it and talk about it today.
He has had a lot of issues with anxiety with school, he went through a period of being reluctant to go to school before Covid had even started, there have been some issues with bullying which we addressed last term (him being the bully)- just giving this background so not to drip feed later if it becomes relevant. But generally there’s no other signs of self harm, he has friends he talks to online, he generally seems ok in himself apart from when he is complaining of belly ache, so why he would want to deliberately make himself sick is beyond me?!
I had tried to look online but all I could find was stuff about bulimia, but he eats well so I don’t think it’s eating disorder.
Can anyone offer me some advice please?