i don't have the answers - just some possible reassurance. my son is 3 years and 7 months old now and no. 2s on the toilet are still hit and miss. he really would prefer to do them in his pants if he had his way.
i know what you mean by letting it out bit by bit. sometimes when my son is on a bad roll we can have four accidents in one day!@
i just say to myself that it won't last forever. he knows it is 'wrong' but really there are worse things he could do. at least he makes it to the toilet for no. 1s 90% of the time and he doesn't need a nappy at night.
your child will get there. just keep doing what you're doing (encouraging good behaviour etc) and he/she will get there!

Mabey try adding lots of fruit to his diet. Also the star chart is a fantastic idea.

Diffinitely a star chart for when he does a proper poo or some kind of reward.  Try getting him into a routine of sitting on the toilet straight after hes had breakfast  or maybe tea, and reading a book or colouring for 5 minutes or so to see what happens.  Don't worry if he doesn't go or feel like going, this will at least give a signal to his brain in case he does need to go and hopefully relax.  Reward him for sitting on the toilet even if nothing happens.  Hes probably a busy little boy who doesn't have time to stop to go to the toilet and he forgets, this is common for little busy bees!  As long as you can get him in that habit of sitting down and allowing his body to relax he should eventually get into the habit of going everyday (all at once!) He may also be constipated, so to get him going try talking to your chemist about something for that, just to get things going.

Good Luck