DS 2 had a stage like this and also a un of sickness and ended up TINY. we put him on 'operation fatty'. added things to his food. we also gave him incremin which helped his appetitie.

some ideas to fortify food and make what they do eat count a bit more:
add milk powder to milk drinks
stir olive oil through pasta before adding sauce
smoothies were great -added yoghurt etc
things that were always a hit - schnitzel (as long as thete was something to dip it in - hummous a hit here), corn on the cob (still his only vegie), fish goujons
I also introduced supper after bath time - this is a hit with both my boys - i usually give them some cereal to 'graze on' some cut up banana maybe and an english muffin with honey and a drink of milk. its like an extra meal that they love