We started my son with sitting down but once the novelty of the potty wore off, he wasn't interested. So then we switched to standing. That went much better. He thought it was fun to stand over the adult potty and pee in it like a big boy. We bought an inexpensive step stool for him to stand on. Aiming has never been a problem unless he is talking while peeing and turns to look at who he is talking to. We initially threw a piece of cereal (cheerios or fruit loops) into the toilet and told him to aim for it but he really didn't need anything to aim for other than the water in the toilet which makes a sound when the urine hits it which is enough motivation for him.
When we are out at a public restroom, if the toilet is high, I either have him stand on my feet which raises him up higher or I lift him up by his armpits. I'm grateful that some stores (like Ikea) keep a step stool in their family restroom.
You should keep in mind that although it would be great to have him potty trained before your next child is born, many children regress back to diapers when a new baby enters the home. For that reason, we waited until my second born was a few weeks old before we potty trained because I figured, why potty train twice. Good luck, congrats on the new baby and I hope potty training goes well. Every child is different and every child learns to use the potty so I'm sure he will learn according to his pace and his preferences.