Hello
I know the feeling - ours has been a fussy eater for ages!
When she developed a liver issue recently, we had to keep her eating to stop it getting worse and the only way I could get her to eat anything was to cook a raw tiger prawn (she loves them) and then whizz it up with some hot water and some Applaws Chicken and Rice food. The smell of the prawn got her so excited she'd eat the lot. It wasnt very pretty - sort of like porridge LOL (and a bit pongy!) but it worked.
Might be worth trying something simlar if theres a treat food your little guy likes? Good luck though - I hope you get it sorted soon!
H