Are you married to the father of your ds? If not, is he named on your ds's birth certificate? .
Have you made application to the CSA for maintenance?
Why haven't you already moved your ds to a nearby school so that he can cement any friendships he may be making with children who live nearby?
Ask your nearest Women's Aid office to recommend solicitors who specialise in neutralising controlling and abusive twunts family law and who offer a free half hour consultation.
If the twunt is able to predict that your ds will want to live with him when he reaches the age of 10 ask him to provide the winning national and euromillions lottery numbers as, regardless of the child's wishes, there's about as much chance of an alcoholic winning custody of a minor as him coming up with right selection.
Bear in mind that his "excessive daily drinking" may come into play when he's making these boasts/threats and talking the talk doesn't mean he has any intention, or is capable, of walking it.
In any event, if he carries on drinking to excess it's probable he'll be incapable of carrying out his threats. To speed the process send him a case of Absinthe for his birthday.
Seriously, honey, don't let his empty threats get to you as the twunt who's a match for the collective wisdom of this board is yet to be born. 