In the short term you could ask your ex if you could see your son at her place ( depends on domestic situation). My ex did that for a bit and it wasn’t a problem.
If your parents are selling up, will they help you with a deposit? You don’t need a two bed flat to be able to have your son. He’s still very young so you could have a 1 bed flat and he could share with you or sleep on an air bed.
If you have your son three nights, your maintenance payment will go down, so that should help a bit too.