I would say, given your detail, that yes, it's too soon. And it may be too soon for a while.
I can also say that I have been divorced just under 8 years, separated for a bit over 9 years and my ex and I had lunch with our respective current spouses and our daughter FOR THE FIRST TIME during Christmas pickup this past month. (We used a mediator for pickup/drop-off and communication for the first year and then ONLY communicated via text/email for the next 5. I can count on fingers and toes the number of voice to voice conversations I've had with him since our divorce). I can also tell you that as difficult as he is to get along with and as much as I abhor dealing with him..... the person who REALLY gets hurt because we can't communicate is my daughter. However, it is what it is. And we opted for NO communication rather than drama and scenes.
So, here is my advice.
1. Don't worry about what other people think.
2. Don't worry that your son will be scarred because his mother was not as his party. PLENTY of kids from LOVING families don't have both parents at all their stuff. Don't make this a measure of your love and he won't.
3. The same parents who are going to judge that you aren't at his party will judge if you are divorced and you ARE at the party.
So, just like you had to get over that some parents hate all your parenting decisions.... this is no different. You have to figure out what works for YOUR SON. period. I, myself, take the *high road* option unless my daughter is hurting..... the stuff that is about "me" (like not wanting to miss haircuts / first leg shaving etc) I willingly chose to throw out the window.... because making a big deal about being the "parent who gets to...." would just make my daughter feel pulled apart.
4. If you and exhubs can't get along together, then you are going to have to do things separately. You, then, have choices to make. Does your son get 2 parties? Or does your son have birthday parties at Dad's but mom is the one who goes to all the soccer games and throws the pizza party afterwards. It's not always healthy to "double up" on everything...... 2 b'day parties.... 2 Christmases.... 2 Thanksgivings.... It's EXHAUSTING for the kid simply because the PARENTS don't want to "miss out". But it doesn't build a positive memory for the kid.
5. Even though you are freshly divorced........ it's never too early to realize that some things are missing from your parenting plan. For example, our stipulates which year each of us has her on her b'day. Now, we now live in separate states so this is actually a bit easier for us. But, if there are large oversights in your decree..... start making a list and you can have this re-negotiated in 6 months or a year. Yes, this will cost you...... so if you and exhubs can work together it's better for everyone.
6. Unsolicited advise not in your post.
Pick a mantra and stick with it. Mine was "Mom and Dad and a Judge have decided this is what is best for you. It's because we love you and we all want what's best for you.... even though you don't always like it and it doesn't always seem to make sense." That's it. That's all she got for like 3 years.
"Why can't you be at my b'day party?"
"Mom and Dad are divorced. Lots of divorced parents do things separately but we both love you. You're going to have the party with your friends at Daddy's. Then you'll have a dinner party with grandma and grandpa next weekend with me".
Keep it simple and factual. Eliminate ANY sign of "I wish that I could" or "this is your dad's decision". Stick with things that are non-emotional. Because your son needs SOMETHING that is consistent and non-emotional.
7. For what it's worth.... you husband didn't turn your friends against you. They willingly went on their own. So...... This is the time for new friends. It's unfortunate.... but at least you have a CLEAR boundary of who is on whose team. So, get yourself some new friends that have NOTHING to do with your divorce. Join a "meetup" group. Get your son involved in play dates that have NOTHING to do with his current daycare - like with a park district or Little Gym. You both need people who are on YOUR side. Find them. Fast.
Here is the other thing that I know beyond a shadow of a doubt. You son will know who loves him. Not by who had the birthday parties. Not by who kept the friends. But by who is able to calm his fears. By who is able to provide a solid structure upon which he can grow.
I wish you luck and peace in this journey.