There were some studies years ago in the US, where they looked at how much boys and girls cried when they were young babies. They found that overall, boys cried significantly more than the girls.
They then tried to replicate the study in the UK and couldn't - baby boys and girls cried exactly the same amounts.
Somebody then realised that one difference between US baby boys and UK baby boys is that the US babies were circumcised. When they went back and reanalysed the data, they discovered that the uncircumsized boys and the girls cried the same amount overall in their first year. Circumsized boys on the other hand, cried for signficantly more time than the other babies. Which very strongly suggests that it is having a negative effect on the boys in that first year...
I'd second what has been said about getting your feelings regarding the circumcision noted very visibly on your ds's medical notes. It's your ds's body - if he wants to be circumcized when he is older and it hurts more then that's fine - he gets to suffer a bit more for his religion and feel more worthy for it, much better than being circumcised early and never able to undo it.
If you do split up with your dh, I'd also make sure that it is one of the things that is documented by your solicitor. So get it written into the divorce agreement that it shouldn't be done, get a formal letter sent to him that it shouldn't be done and that if it is, you will be reporting him for GBH (or whatever the solicitor says the appropriate charge is) and whatever else they can suggest as safeguards.
I'd also ensure that you have as much evidence as you can get about how many muslim traditions your dh doesn't follow so that you can argue along the lines of why can he choose which traditions to follow or not - this isn't about not eating meat or going to the mosque each week, neither of which will have a permanent effect on your child. This is about unnecessary mutilation of your child and it's absolutely right that your ds should be allowed to make that choice himself once he is an adult.