First, it's very common for toddlers not to eat dinner. Or eat 2 bites and be done. And that is perfectly okay.
Read the book 'How to get your kid to eat, but not too much'.
I am against forcing my kid to eat. She's almost 18months and if she chooses not to eat at a mealtime - its fine. I don't make special meals and I don't argue with her to eat what's on her plate.
She usually doesn't get her milk until she's had a bite or two, but if she's flat out refusing and screaming for the milk, then fine - she gets the milk.
I just make sure that her daily options are healthy and I relax because she will not starve herself and I refuse to make food into a control issue.
Dinner will commonly be the smallest one they eat. They're tired, they don't need the energy - breakfasts are commonly larger - they're hungry and they need the energy to play.
Now, if you created a bad habit with bagels, I suggest giving whatever you made. If he refuses to eat it, let him down. If he wants something 10minutes later, offer the same meal he just had. He will soon understand that you're not a short order cook.
If he continues to refuse until the next meal, offer the new meal then.
SAHM to Magnolia May (09/10) and Luke Russett (04/13) and wife and best friend to my airman.