I did not use a potty for my 2 boys at all, didn't even contemplate it. I bought a proper toilet seat insert that goes under the seat but fits perfectly for a childs bottom so he does not have the feeling of falling in.
I bought a small step stool with a grippy bottom so it wouldn't slip. So they had not problem getting up to the toilet to either sit or to stand for a wee.
To encourage him stand up for the wee, put a ping pong ball in the toilet and see if you can get them to aim at it, it will roll with the water movement and is a great game, the ball will not flush as it's full of air and light.
An alternative to the ping pong ball is to put a sharpie mark on the actual bowl and see if he can target it, a bit of metho will get the sharpie mark off later.
I found gently simmering 4 prunes in a cup of water on the stove for 15 mins and drink the water, also with eating the prunes great thing to help with the BM. Grated apple and mashed bananas with yoghurt is a good one also.
Summer is a perfect time for toilet training as they just get around in undies and you can pop them off to the loo in no time, no extra clothes to contend with.
Only once did I have a problem with ds2 and constipation, came and went just like that!
Good luck.