My DS did exactly the same with fruit at the age of 3 or 4 and still won't eat any fruit unless it is orange juice (which he has watered down) and cooked apple, as in apple strudel or crumble. Unfortunately, I thought it was a phase and he is now 7 so I don't know when this one is going to end.
I really would like to know if something happens to their tastebuds at that age that makes them seemingly hate all the stuff they used to like.
The only thing I have been able to do it make sure that he eats as much veg as possible to compensate so I suppose you would have to make sure that your ds eats as much fruit as possible.
Hiding fruit is very difficult it seems and my ds is not easily fooled but I think others are right about hiding veg in things like spag bol or cottage pie as it seems easier to do with veg. Try not to hide too much at a time as he will notice. Don't forget also, that baked beans are also a veg although obviously you can't live off those for ever. We tend to have low salt/sugar ones.
Will he eat salad such as cucumber and tomato? They are not so obviously vegetabley and every little helps.
The other thing I have seen on a healthy eating programme on telly is to offer a treat if they will eat just one mouthful of something and then start upping the amount very gradually. The star chart might work for this. Don't be over ambitious though and try and get him to eat a whole portion, don't even put a whole portion on a plate, just one mouthful at a time.
It doesn't work so well with fruit though sadly, as by the time my DS has eaten his main course he can take or leave the desert so no amount of bribing seems to work.
I hope you get your DS back on track soon.