he would like to go through mediation, but after reading so many responses here, I feel that I can't trust him.
Look, every situation is unique and personal. My opinion is that unless the two of you are at complete disagreement, then it's in your best interest to agree on your settlement either on your own or through mediation. File for divorce as you've already planned. This just starts the process and puts you in the driver seat. To ease the tension tell your WS you filed to simply begin the process and wish to attempt to negotiate a settlement.
If you are able to, then discuss and negotiate the terms with your WS - children arrangements, financial arrangements, housing arrangements, etc. If you are on the same page, then have your lawyer document it as a settlement agreement and send it to him for review. Make sure to have your lawyer review everything you discuss and do NOT sign anything until you've had an opportunity to review it with your lawyer.
If you're unable to have that conversation on your own, but still think an agreement can be reached then schedule a mediation.
Others may disagree, but the goal here is not to win everything - reasonable compromise will be required. Make sure you know what is important to you and what you are willing to give up. If you're too stressed to know, I suggest talking it through with a counselor or friend.
And if you are both at complete disagreement, then prepare for a legal battle. But understand that it will be expensive and drawn out... Making the first two options seem more attractive.
Also, discuss with your lawyer any options of not serving your WS with papers. Depending on your state, you may be able to save some WS embarrassment and ultimately lesson the tension if you serve them to his lawyer instead.
Good luck and be strong. The light at the end moves a little bit closer.