First, he may not yet be ready, so stay calm, keep encouraging and try not to punish him.
Have you tried letting him stand to pee? Have dad show him how a couple of times. You will need to let him stand on a step stool to reach. Oh and have plenty of disinfecting wipes for when he misses.
When we switched my son, now 4, from sitting to standing he did so much better and was out of diapers and pullups in a couple of weeks. Also, don't put anything in the toilet to have him aim, that will only encourage him to put things in that you don't want him to. Just tell him to aim at the water so that you can "hear it." My son still likes to "hear" it so we don't have too many messes.
Get one of those soft seats that sit on the toilet seat for when he does have to go poo poo. They come with a splash guard, and are a lot easier to clean than a potty.
Good luck.