hi, i too have this very same issue with my dd, who is exactly the same age as your ds....my dd weighs 2 stone 5pound!!!
After being very ill for 3 weeks she looks terrible, my sister hasnt seen her for months and saw her last nigght abd cried, as she says she looks like death!!!! her eyes are sucken into her head and are black and her cheek bones stick out, as do all the other bones in her body.
i too cry at the poor sight of my precious girl, its music to my ears when she says 'i am hungry'.
i have been giving her 'complan', recommended by gp, its a strawberry shake drink, i mix mine with full fat milk, get it from chemist at around 3 pound.
also i give her what she wants to eat when she wants it basically, i am probably gonna get shot on here for seaying that but....whatever, my dd can cope with a little more weight on her.....she is soooooo active its amazing, gymnastics, dance, drama,swimming etc.
let me know how you get on as its very interesting to swap stories.....good luck and remember you are not alone.
someone said to dd yesterday at school....'god, you still look so ill i dont think you will ever get better'.
i just smiled and carried on, it broke my heart but they really didnt mean any harm.
keep posting, i will get back to you.