My divorce (mid 80's) was quite amicable and both of us behaved like grownups - well for the most part. But I've had friends, both male and female who've dealth with situations that sound somewhat like you've described. A former partner who at first seems reasonable and fair and then starts saying and doing things that can cause great harm to everyone. At that point I always offer the same advice: Talk to everyone you know and get the attorney who can if necessary turn into a SHARK to protect your rights. No, I'm not saying you should take him to the cleaners or get sole custody of your son, I'm saying get someone who will make sure you are treated equitably and that your son's best interests are looked after. I've seen good attorneys and I've seen bad. Get recommendations from people who've been though tough divorces and call the best attorney you can. It WILL be worth it.
BTW, I'm so very sorry you have to go through this. An amicable divorce is hard enough on everyone in the family, a horrible one is almost unbearable. But hang in there.