Pay attention to what he likes and then try to capitalize on that. If he likes crunchy things, then maybe he would like a "sandwich" made on crackers instead of bread. Vegetables with dip....sometimes the dip is all it takes to get los to try.
Also..what if he could help you make a sandwich? For you, not for him. If he helped you make a PB&J, and with no pressure you offered him a bite "Wow this is SO good! You make the BEST sandwich! Wanna try it?"
Lastly...what does he say about it? If you said "I've been thinking that I really don't want you to be hungry at kindergarten in the fall! Do you have any ideas about how we could make sure you bring enough to eat in your lunchbox?" What does HE say?