While it's great for Dad to be involved and around...at some point there will need to be more defined bounaries. For now, if you think there's any chance of reconciliation, you don't want to get too definite on "visitation" in the event he comes back and this is just a "phase". But I wouldn't think it unreasonable for you to suggest to Dad that you guys plan a little in advance so you can plan dinner, activities, etc. Also, he should always call (or make arrangements) in advance, not show up unannounced, knock & wait for an answer (not use his key or walk right in).
You need time with your son as well and your son needs a somewhat stable routine he can count on. Also, you need to be able to plan for your free time and use it wisely.
Counseling might not be a bad idea. Even if your husband has made up his mind, it's important that you guys are in sync as parents.
Don't be afraid to stand up for yourself and your son - but pick your battles. The more you're able to rise above the situation (both of you), the sooner the healing begins and the better you'll get along going forward.
I was the iniator in my divorce. My husband made the decision to move 1,400 miles away about a month after we separated and moved 5 months later - our son was 4 at the time. It's taken a long time for both sides to heal but my sticking to my guns about what was important and rising above the situation as much as possible has contributed to a much more positive situation. I eventually moved so my son could be closer to his Dad, grandparents and my family. We still don't necessarily approach things the same but my ex has seen that I always try to do what's best for #1-our son and #2-me. I have not put "winning" ahead of doing what's best for our son. And I believe that has caused my ex to trust and respect me - which is difficult after going through a divorce which pretty much disintegrates trust.
If you get to the point of needing an attorney, I can recommend one. I haven't used her (I was in MD when we divorced) but she's a friend and she approaches the process as positively as possible. Also, be careful about who you vent to and/or get advice from. If you're talking to your best friend who is angry at your husband because of the situation, she might encourage the wrong response. If you're talking to your grandmother who thinks you should be singing a Tammy Wynette song, that might not be good either. Try to talk to folks who sincerely care about you and your son and will respond in a balanced, productive and positive fashion.
This too shall pass. Not as quickly as you'd like - but it will pass. Meanwhile, continue to ask for prayers and grow in your relationship with God.
Peace,
C.