Do you sit at the table with him coaxing every mouthful? Busy yourself doing something close by but not hovering. Inviting someone round with a slightly older child could work wonders for stimulating appetite. Someone his own size to copy. And your own enthusiasm for food on your own plate can help.
Try varying cheap beakers and plates, even his place mats. Have an occasional indoor picnic on the floor.
Instead of "No" try a "Shall we...?" or "Hey let's...!" with enthusiasm. He will tune out "No" so work round it.
Daft little rhymes, songs, a kind of cheer leading. And when you run out of steam, CDs or tapes or radio.
Toddlers don't always appreciate lots of choices, so don't fall into the trap of bewildering him with an array of alternatives; ironically, he'll hate rejecting anything.
At home as he gets older, using an egg timer to see who wins and how fast he can accomplish something can be powerful motivation.
Distraction, warning, carry through consequences. Never threaten leaving an activity if you're not prepared to cut it short. Be consistent. Consequences should be immediate not some forfeit hours later.
If physically difficult to bodily lift him, remark that it's a shame he is being silly because he's going to miss out on X. If he shows no sign of budging, you can wait all day, nowhere else to go, this is boring but he'll miss out on Y.
Never be embarrassed by onlookers tutting!
Or studiously ignore him, walk on a little, covertly keeping an eye on him, not addressing him, not bargaining.
Your best bet is to try and anticipate tantrums, you're going to recognise a lot of potential booby traps - a chore that involves him keeping quiet, crowds, everything above his eye level, outings when he's tired, or ready for a snack to re-fuel.