He also says 'how many mouthfuls do I have to have' pretty much straight away when we sit down
Just tell him it's up to him and change the subject.
You need to stop thinking in terms of wanting him to eat full stop. It isn't your job to want him to eat or to worry about whether toast is too nice a treat to have as an alternative.
You philosophy needs to be that you will provide a balanced diet and he will choose from what is available. If he would be allowed toast as a snack after having eaten a meal, why deny it him because he hasn't eaten? If it wouldn't be available after he's eaten a meal, don't make it available because he didn't eat.
If he's hungry at bedtime tell him there is fruit in the fruit bowl and engage no more than that.
TV and other nice activities need to be available because they are available and not linked in any way to food.
If he messes around at the table tell him that, if he would like to sit at the table, he needs to behave nicely and, if he continues to mess around, he will have to get down and wait elsewhere for the rest of you to finish. Once he's down he doesn't get to interact with those left at the table or get back up and eat any more.
Don't praise him for eating or express any displeasure at him not eating. What he chooses to put in his mouth is solely his business. Don't offer food but allow him to select what he would like from serving bowls on the table. At the end of the meal ask him if he has finished and take what is left away while talking about something totally unconnected with food.
This is not going to be solved in a day. You need to change your approach and then allow him time to realise that you are behaving differently, change his behaviour a few times to see what effect it has on you and then, when he understands the new ethos, settle into it. Be consistent and patient.