I assume you use breakfast and after school club as you work. If you are a lone parent and working you should get help with your childcare costs if they are Ofstead regsitered (which they should be if attached to school)
So you can get help of up to 70% of your child care costs if you apply via your WTC/CTC claim. Look into this as soon as possible. You should also be able to claim some level of WTC and CTC unless you are on quite a sizeable salary.
If the afterschool & breakfast clubs aren't ofstead registered then you may be better off finding a child minder who is.
Ex should be paying maintenance towards ds. Even those on IS pay £5 a week. It is one of his responsibilities as a parent to financially support his ds. Sadly some NRP use maintenance to get back at their ex and think they are hurting their ex by with-holding it without thinking of the inpact on tehir dc.
If he isn't declaring his income from his flat (is he even allowed to sublet it?) they you can inform HMRC that he has undeclared income. If the landlord isn't aware he is subletting then maybe you should bring that to their attention. If it is a council property they won't be impressed at all.
He is not witholding money from you but from his ds. It is your responsibility as a parent to ensure ds is provided for, and that includes ensuring your ex pays his share towards his upbringing. So don't buy into his guilt trip that you won't get him in to trouble because it would 'harm' ds.
By not contributing he is harming ds more, and you won't be any worse off that you are now by reporting him to the relevant authorities.
Re contact you don't have to get him to agree. Set out a reasonable suggestion of contact, stress to him the importantce to ds of regular contact, and ask him to confirm he agrees to your suggestion and give him a timeline to respond.
If he is not happy with it ask him why (maybe work commitments etc you are not aware of) and what he propses instead.
If you can't agree then suggestion mediation as a way to deal with it.
Don't agree to last minute or ad hoc plans, ask him to stick to the agreement and if he won't then he misses his contact. Yes it can be hard oon your ds, but if you don't tell him his dad is coming then he won't get upset if he doesn't show up.
Until you put your foot down and make a stand for your ds's sake then he will keep messing you about.
Make sure you put your proposal in writting,, keep it factual and unemotional and keep a copy so you can show you have been reasonable and are not withholding contact should you need too. From what you say I doubt he would take it to court though.