Argh! Please help with advice in feeding 16mo DS!

[tinkerbug]

DS is 16mo and has never been a good eater. We weaned at 6 months ish and as soon as he started dropping bf his weight went from 50th percentile to 9th.
He has 3 beakers of milk a day (first thing, mid afternoon and at bedtime, about a pint a day). He loves his porridge for breakfast and shovels down loads but won't try anything else (eg Weetabix) Lunch and dinner a disaster, he will only eat toast, mini cheese sandwiches and chicken soup. He eats better at cm but here is a nightmare. Tonight he has rejected spaghetti and also veg soup. Dh says not to give him anything now as he needs to learn. Is he too young for this? Do I just keep trying and let him go hungry when he won't eat what's offered? Or do I give him anything he will eat (always eats rice cakes, breadsticks and yoghurt!) Not sure what to do for the best?!
Apologies for mammoth post, any help much appreciated!

I was worried about DD1's weight so I went down the route of giving her what she likes to a certain extent. Then I looked at what I could always get her to eat, and then worked out ways of bulking out the calories - so if I was you I would add something calorific to the porridge like cream/unsalted butter/mascarpone. Butter on veg, cream into the soup, full fat yogurts etc. I think he's probably a bit little yet to give him nothing - he wouldn't really understand why. Its about getting something into him I think at this age (my DD1 is 22 months). The way we play it is to always offer a bit of what we're having (I refuse to cook 2 meals). If its not eaten then she gets some bread & butter & yogurts (that being standard pudding here). His eating will improve eventually I'm sure. The other thing I do is give her vitamin syrup - I buy it at the supermarket and pop it in her water. Our local health authority recommends it here, & if they get iron deficient, then it can be linked to a loss of appetite. I tried her with horlicks as that is fortified with lots of vitamins, & I thought she would like it but no cigar. Ovaltine is also fortified if you think he would try that?

Thank you JetLi, some brilliant ideas! I think vitamin syrup might be a good idea just to check he's not lacking anything. Will also try the Horlicks / ovaltine suggestion! Thank you!