You are asking two questions:
1) Is this necessary? Maybe, maybe not. If your child's head has always been big (this should have been measured all along) and is growing proportionately, then your health care provider may be overreacting. My son has a big head (> 95th percentile). Guess what? I have a big head. My husband has a big head. We're both > 95th percentile. The apple doesn't fall far from the tree. So maybe the doctor needs to look at the heads of you and the father of the child, if that is possible. If you are uncomfortable with this, you can always ask for a referral to a specialty clinic or children's hospital. There are medical problems that can cause changes in the skull size and it's okay for you to have more knowledge before you agree to this.
2) MRI sedation--yes, as a parent, it is uncomfortable to have your child sedated. Ask questions--who is doing the sedation, what is their experience with children--and if you are not getting answers that help you, try a larger hospital with a pediatric wing or a children's hospital. The reason that your child will have to be sedated is that he will need to hold very still. The MRI scan machine is very noisy and any movement ruins the images.
Finally--that is one cute baby boy!