you've already admitted one challenge - is your consistency in offering rewards. It is confusing for him.
Here are some ideas: I posted this to another gal who has having similar challenges:
3 1/2 is NORMAL to still be learning - especially for a boy. You did not mention if there are any siblings. Consider changes that may have taken place in routine or additions to the family.
Some tricks I learned with my son .. who still had occasional accidents at 4:
see if his friends' parents, or his cousins' parents, will let him watch them go. Peer pressure - or just seeing someone his won size using the toilet will help much better than seeing daddy or mom use the toilet.
With my son I put a blue toilet cleaner in there.. and he liked making the water green - it was a game.
When he does acknowledge that he needs to go: put one small square of toilet paper on top of the water so it floats, Make a game out of it and ask him to make it sink.
Be consistent with incentives: 3 small candies (kept in a small jar out of reach in the bathroom) for each time he succesfully goes.. and after the routine of washing and drying hands.
Big toilets can be difficult, provide a stool for him to rest his feet on . and to reach it. There are special seats available to make the seat smaller for when he needs to use it.
If you have a training toilet.... keep it out in the living room or closer to where he conducts his activities, to remind him what he should do when he feels the urge, and to have quicker access.
Stickers - a sticker for each day he is accident free (you might not get to do this until later) and offer a prize for one week's worth of sitckers on the calendar - and put a picture of a toy or activity he wants near the calendar and look at it every morning so he can be reminded of that motivation.
Cheers and clapping when he succeeds... a little less negative attention when he doesn't... "maybe next time you'll remember."
and let him go naked for a day.... some kids are more aware of when they need to go this way.. this is how my oldest daughter potty trained.
also.. there are pull ups that have features to help provide incentive too... designs that disappear when wet, feel cool when being made wet, designs that well.. they may just not want to pee on!
there are many things you can do... but your positive praise.. and less negative attention when he does have accidents.. may be the best thing.
but be consistent.. it will take a while.. boys generally take longer, and most kids will have set backs... hang in there.