Age is certainly a factor in this process. So is consistency--so it would be better if your son's father was a little more in tune with what you are trying to do.
Pampers does make pull up diapers that are really disposable training pants. It might be good to switch to those and abandon the use of regular diapers. Toilet training does involve relinquishing the use of diapers--and some children are anxious about doing that, and so the whole toilet training process can get slowed down. Shifting to only using the pull up diapers (training pants) might make the situation a little clearer for your son. It may make it easier for him too, since it will break the toilet training into "stages"--the first stage being at least able to say bye bye to regular diapers. He may also be more reluctant to have an accident in the disposable training pants--and that might give him a little added incentive. And the training pants may also give him a feeling of more independence--and that might be beneficial too.
Why are you concerned--at all--about whether he pees sitting down or standing up? At this stage of the game all you want him to do is to recognize the urge to go, signal you about it, and to try to hold it until he gets to the toilet. If he wants to do everything sitting down, that might be easier for him right now. He can copy you right now--he doesn't have to copy his father.
Actually it sounds like you are making excellent progress. Many children regress or have accidents--that's pretty normal.
Just keep encouraging your son to let you know when he has an "urge", and to try to hold it until he reaches a toilet. And praise him lavishly--make a really big fuss each time he succeeds. Just downplay any lapses or accidents--don't scold or act really upset about them. Positive reward and encouragement will do the trick--given a little bit more time.
I think what he said about the maxi pad was adorable--you have a bright kid It also gave you the chance to say, "I don't want to have an accident. I prefer to use the toilet".
That's the message you are trying to get across to him anyway.
Just have patience. Most children are toilet trained by the time they graduate from high school
They also sell special potty seats--some play music when the child urinates sitting on them--specifically to help with toilet training.. These are designed to give the child positive feedback for using the potty. You might look at those in a store and see if they might be useful.
But I think that time and patience--and lots of praise for each "success"-- will do the trick. It sounds like you are already on the right track.