I faced this issue 12 years ago (it was sort of the turning point in me becoming an intactivist rather than simply a parent who chose not to circumcise)
I did it all wrong (because I had NO IDEA what I was up against)
I thought it was about the baby.
For me this was not a question or a decision- this was not going to happen and it might have broken up our marriage if it came to that- it almost did.
I don't know how the subject got brought up, but I think I might have just offhandedly assumed that we weren't going to circumcise because DUH... you don't cut off bits of other people's penises.... and WOW- I was not prepared for the total WALL of defensiveness I got. "You must hate me" etc... it was all about him and his sexual ego and the fact that I was supposed to prove that he was perfect and awesome etc etc... by doing what had been done to him to the baby. If I respected the WHOLE male, based on the fact that I knew whole men and respected their foreskin as a part of their sexual anatomy - (like full of veins and able to feel sexual pleasure- and as such it was precious not GARBAGE...) oh the hell opened up.... how dare I acknowledge sexual anatomy he does not posess. I might have well called him an impotent sexual cripple. He was irate. This was not normal behavior for him- but it was like I hit a trigger that just flipped him to total irrational maniac who coluld not be reasoned with. We fought for days or weeks... I don't remember exactly... how long does hell last? I was on bedrest- I had placenta previa... I was praying I'd deliver a living baby of a survivable gestation and he was planning of cutting up the baby's penis as soon as it was born just to prove he wasn't sexually damaged. One of the fights included a total primal meltdown sobbing in the fetal position insisting that he was not damaged. ... Whoa. (at this point in the tale may I sugged reading Ronald Goldman's book, Circumcision: The Hidden Trauma)
I don't know what happened exactly behind my back, but he looked into it himself and something must have gotten through and he begrudgingly softened his stance. The night our son was born (by the grace of God we both lived) another instinct kicked in- his previous desire to defend a past circumcision became a firey desire to protect his beautiful baby. He didn't REALLY get it till he was standing in the bloody OR holding our new baby.
Years later we found out two additional important facts. 1. His father was intact (he'd argued a boy must look like dad) 2. His parents did not chose to circumcise him- it was done as part of the hospital routine and had his parents even had the option, they would have said no. This story was never relayed to him because... well, typical shame and secrecy surrounding this issue which is everyone's business but the man who has the penis.
So here is my advice to you, if you haven't already. Leave the baby out of it for now. Leave your own ideas out of it for now. Begin by asking lots of questions and together collecting answers. The object is not to be making a circumcision decision- it's about first trying to reclaim a little objectivity so that you can approach this question without the burden of "circumcision is great because I'm circumcised, to question circumcision is to question me and I'm just not going to let you go there."
One thing I'd suggest doing is talking with the inlaws about his circumcision. You aren't asking their advice- you are getting the history. Know the reasons and whatever the reason is, there is an empowering way to question that reason that builds the circumcised (infant) man up. Try to put the event of that infant circumcision in the context of the time, the information given, the empowerment of the birth process, the decision making model etc. If possible go back another generation to his grandparents as well.
I also think that it's wise to ask him to research it and to ask him to make a list of his concerns. Then address his concerns together one by one.
If you begin with your opinion, you may just begin a dynamic of defensiveness that goes no where.
What you need to understand that one of the reasons why circumcision is so powerful is because circumcised people NEED their beliefs to cling to in order to avoid facing the reality of what was done to them. Thinking about circumcision as sexual damage rather than the sexual advantage is really painful! I suggest concepts presented by James Atherton on Learning as loss, Resistance to learning and superlative learning. In order to acquire new understanding requires a person to enter a destabilizing period of letting go of previously learned beliefs. http://www.learningandteaching.info/learning/resistan.htm
Before you begin to discuss the baby you must discover the truth about his beliefs about his own circumcision.