OK apologies in advance if any of this seems patronising or is teaching you to suck eggs - it's not meant to be! It's difficult explaining this as flat advice as usually I would spend about an hour doing an assessment with the parent(s) and child.
Most children that won't use the toilet do so because they have created a bad association, either through having done a painful poo or a sore wee, or because they are just far too busy to sit and use the loo. They then started to hold on to wee or poo, which means that the bladder has probably got overstretched and flabby and so can't hold much wee before emptying, leading to wetting. Continued wetting means that the poor despairing parent resorts back to nappies/pull ups. This means the child feels that the pressure is off, so no need to try sitting on the loo, and then wearing a nappy means they just wee whenever they want to so still aren't developing the means to tone and strengthen the bladder. It's a vicious circle.
Holding onto poo is also quite common (especially in boys apparently) and again causes the rectum to stretch meaning that it holds more poo before emptying, causing huge poos which can be quite painful to pass. Also as they have been held in the rectum for a while they are drier and again more painful. Usually giving a gentle laxative-type medicine such as Movicol Paediatric gets things moving so that they can't hold on and as the pooing is more comfortable the fear is eased.
A toileting routine is a good way to help tone the bladder - the bladder is a muscley bag and like any muscle gets stronger the more it's used. A strong toned bladder holds more wee and empties more efficiently, as well as sending a stronger 'go for a wee now' message to the brain, making it harder to hold on. The child should be drinking about 6-8 drinks a day (of about 150ml ideally). If a child is reluctant to drink (as many preschoolers are) let them drink pretty much what they like within reason; gallons of milk or milkshake will reduce the appetite and also milk is a food so takes longer to convert to wee.
Start planning a toileting routine that will fit around your lifestyle, eg ok when we get up in the morning you are going to sit on the loo for 1/2/3 minutes (use a timer if necessary), then again before we go to school/preschool, then again at preschool at say 1100, then again after lunch at say 1300, then again at say 1500 and so on up until bedtime. Sitting on the loo is non-negotiable - it's REALLY hard I know but try and enforce it. Use bribery, books, DVDs, whatever it takes to stay there for the allotted time. Maybe start off with 1 minute then work up to 2 and then 3. When the time is up you reward the action ie sitting on the loo, and NOT the outcome ie whether he weed or not. Sooner or later some wee will come out when he is sitting there (thanks to all those fluids!) and you do the hurray yippee doo dah routine and give an extra sticker or whatever. Have a pre-agreed reward such as if you have 6 stickers by bedtime you can have an extra story or 10 minutes of a DVD or whatever works for your child.
It can take some weeks but if you are really consistent IME most children are reliably using the loo and are out of nappies (excluding SNs or a physical reason) within a month. Also remember that peer pressure is a wonderful thing and seeing all the other boys and girls using the toilets at school can be quite powerful.
Also if he is starting Reception in September he should be having some previsits to the class this term. Have a chat with the teacher; I would be surprised if they assign someone to change him but I am sure they will work with you to continue the routine you have been doing at home. It's always worth giving his teacher a heads up as to the difficulties you have been having so s/he can be alert to reminding him to use the loo, drink his water bottle etc.
Sorry that's so long, hope it's of some use for you.