He's smoking crack. I assume there is no order in place at the moment and this current arrangement has been in place since you split?
How old is your son?
Has there ever been ss involvement?
How far away does the Ex live?
How long has he been with the gf?
Where are picks up/drop offs done - at your houses or school so you don't have to see each other?
If you find him a bully, communicate with him via email. Ask him to put down his concerns and what his proposals are.
Not necessary at this stage to instruct a solicitor. Save your money for when you really do need one.
If he IS going to make an application he has to show mediation has been attempted but there's nothing stopping either of you engaging before an application is made.
If you end up in mediation remember, what agreements are reached are not binding but it might open his eyes(and yours maybe) what realistically can be achieved.
Arm yourself with as much info as possible beforehand other than relying on MN(where the views/advice can be polarised)
Do you have any friends in separated relationships. Do you have mutual friends?
Ignore the gf, and what he says the gf says...she'll only wind you up.
He may make progress, he may not - these are some of the factors that will affect whether he does - but, he wont get what he wants.
You
Him - his emotional intelligence is key, already we've had an insight.
your communication to him
His communication to you
Age of your son
The status quo
Valid welfare concerns
mediation
If you self rep,
If he self reps
Your barrister
His barrister
The judge who hears 1...n hearings - where n can be a large or small number, and you may not get continuity of judge.
Whether Jupiter is aligned with Mars.
And most important - WHATS IN THE BEST INTERESTS OF YOUR SON.
All of these would have to be in alignment if he had half a chance.
You are a long way from the above so don't sweat, and deal with each step as they come.