Have you tried steamed broccoli? It’s always been my daughter’s favorite
I’ve been making bone broth and adding Chayote(don’t know the name for it in English) to it since it’s currently his favorite veggie! I notice he loves his veggies in soups or stews
If he’s a sauce lover then you can mince a bunch of vegetables and add it to tomato sauce.
I also put spinach in muffins and pancakes that I make
I make lentil soup for my 13 months old and put all veggies,he loves that