Just wondering if he has reflux? My LO has it - causes gagging, retching, puts him off his food - he has a very poor appetite too. Reflux can be silent (ie with no actual vomiting). Vomiting can also become habitual when it happens so often, not necessarily a deliberate behaviour but a learned response. Reflux can be treated fairly easily, with a combination of medication.
We had a SALT referral to rule out an over-sensitive gag & they were also able to advise on play-related food activities which helped him enjoy his food a wee bit more.
He is now on the dizzy heights of the 2nd centile (started off below th 0.4th) so it's been a long slog. He's still not particularly interested in food, but is getting better.
If he likes yoghurt, a good thing is marscapone mixed with fruit puree - very calorific but tastes just like yoghurt. Or else you can add double cream to ordinary full fat yoghurt. Also use fatty/luxury ice-creams, rolo/milky bar desserts, aero pudds, cream in soup, cheese for a snack - plus cheese straws, raisins, chocolate raisins, peanut butter/philadelphia with breadsticks. I cook all his meals separately in bulk & freeze individual portions. Favourites are macca/cauli cheese, fish pie (all made with Jersey milk, double cream) spaghetti carbonara etc
We give him a toy to play with at mealtimes as a distraction.
If he'll only take small amounts, the idea is to make his food as calorie-dense as possible & limit low calorie foods eg fruit & veg
I think ABIDEC is only vitamins, not minerals
It can be pretty stressful having a poor eater - remember you're only responsible for the food offered, not what he actually eats