I would prob try multivits anyway as prob can't do any harm.
With ds he developed a real food phobia so we just tried to leave it around adn get him to eat almost without noticing.
So I let him eat in front of telly (not good for some as distracts them too much).
Let him choose food from the shop (to eat there and then if necessary) -not major stuff. Ds likes ciabatta rolls (ponce) but if he's not eating much that day, he will have lost interest in it by the time we get back (5 mins walk) so I let him eat it as soon as we have paid.
He gets a packet of chocolate buttons or jelly tots a day if his eating drops again. They're not great but give him at least some short term energy.
Only give little bits at a time - not a whole meal. Ds eats his food in stages. So he'll have a yoghurt, then a piece of toast and then a piece of fruit.
Leave snacks around if you can - grazing can be good.
You can tell if ds is going to have a bad day as he won't eat breakfast - I just ask him if he wants anything and then leave it if he doesn't and ask again around 10.30.
Don't stick to mealtimes, offer food every hour or so. And leave it there.
Get what he likes within reason - ds likes crisps so we get salt and shake and take the salt out. He also likes McDs breakfast so I take him there if he's having a bad run of days.
Does he have any particular issues with types of food? Or just a general lack fo interest?
Cooking may help, doesn;t with ds as he is interested in cooking - but that doesn't follow thru to actual eating.
We do picnics in the front room - all laid out and he helps me buy the food beforehand.
Never beg them to eat or make issue as hard as it is. never comment on how they are eating it.
eat the food he definately likes in front of him and offer a bit.
Basically I try to take it as laid back as I can. He can eat when and where he wants. but he will rarely if ever ask for food so has to be prompted at all times.
If he is worse in certain situations at eating exclude them for a while - ds used to be terrible if you made him sit still at a table or if he was out anywhere, or if we had people round. SO I avoided all that for a while.
At least my ds is cheap to feed