I would definitely advise avoiding salt but as well as sweet and savoury there is sour flavours. So citrus like lemon, Greek yoghurt (not flavoured) etc. maybe try that? Also you can lean into sweet veggies? My LO loves butternut squash puréed and mixed into orzo - it’s sweet for her but gets the veggies in too 😊
Keep offering, it can take up to 20 exposures for them to accept some foods. And I agree that you can try seasoning with different herbs and spices, or try cooking the vegetables in different ways eg roasting rather than boiling to make it taste more interesting.
Do you add seasoning I.e salt to his food? My girl loves salt which helps her eating more solids. Obviously not high amount, but she enjoys food when it’s salty, so I always add a little bit to the ones I buy from the shops as they don’t have any.