Do a star chart- make it yourself. Make it a big one- A3 or A2. Blue tack it up in a prominent place- fridge or hallway. Do it for one week to start with. Days of the week down the left hand side, then stars (or any stickers) are given any time that a wee is produced on the potty. (If child is not getting hang, you can start with a star for sitting on the potty never mind if wee is produced, then additional star for wee.)
This worked really well for me both time- gives immediate reward- actually we didn't do the chocolate buttons with DD just the stickers (did do them with DS, but DD very ready and happy with stickers only!), although I did have to resort to ice cream bribes for poos but that's another story.
Having the chart means you can see how much success you are having as the week goes on, and then make a new one for the second week if necessary. Think DD only had chart for one week as it kicked in really quick for her, whereas DS had one for two or three weeks.
Ah ha, I think you need a "poo book". I had one for both kids as they were both reluctant to poo on potty/toilet (toilet is recommended, less mess). DS we had one from library called "Who's on the loo" and DD we bought "Everyone poos" off amazon. Both fab and worked so recommended!
Also, I would recommend just putting him in pants, not pants and trousers for the first 2 or 3 days- or longer if it takes longer for him to get the hang of it. This saves on washing loads of trousers and is easier for them too- well my kids found it easier. DS was wandering around in just pants at home for a week or so, DD only a few days.
good luck! Sorry for long post!