It can sometimes feel like the battle is lost over these things, the reality is there needn't be a battle over food. You are a team of two, he is the player and you are the manager. These formative years are your responsibility, life will be much easier for you both later on if you can take on little issues in the early years.
When he is hungry, he will eventually eat if it is there and available. Offer him choices, less of the one he likes, more of the one he doesn't. You will have a few meals that are a write-off going forward.
Eating grot, his little system will go into overdrive and you will soon (or now) have behavioural consequences too.
A lovely list from @YappityYapYap, and some great healthy and very popular other foods from other contributors, @LuluJakey1, @IHaveAMagicBean. Try to wean him off the bad things, and fundamentally (and I also concur) try not to have them in the house. You are still in with a good chance on day three, but this could be a really nasty habit to break come day 30.
Diarrhoea can be common consequence for toddlers who overdo certain fruits and veg, but mix these with carbohydrates and his system may tolerate them better.
Don't forget about drinks too. Plenty of water is a great habit to get into early, and go for a receptacle that he likes, 'sippy cup', beaker etc and just keep offering it to him with some insistence. Drop back on any drinks that might be keeping him going, such as reducing any morning milk in order to get his breakfast appetite back to where it should be.