Hi K..
Alright... I've read all the comments... and I am certainly going to be the lone voice here. But why does he NEED to give it up? He only has it at night and at bedtime. That seems reasonable to me.
You have a 3 month old. For the last 3 years your son has the been the center of your and your husbands attention. He was the only child. Now, he has to share his most prized possession... his mommy and daddy. Mommy and Daddy are everything to him ... they are safe, they are secure, they provide him with everything he needs. Now, you're forcing him to give up his SECOND most prized possession... his paci. Seems like a lot for a little guy to handle. Even at 3 kids are still trying to work out how they feel and handle their emotions. His whole world has been turned upside down. We can explain about the new baby and all of that...but in the midst of 'needing' comfort... it's hard for kids that age to understand that mommy or daddy can't come right away, or hold him, etc...
It seems like he needs the comfort. It isn't doing any harm quite frankly. Most dentist will tell you that as long as they give it up before permanent teeth come in, then there is no dental harm.
I'm all for teaching our kids to be age appropriate and be 'big kids' ... but why strip him of his comfort item when he obviously isn't ready and has so much to adjust to. Think about it... YOU have some sort of comfort item when you feel upset ... we all do. Whether it is our morning coffee, a particular sweatshirt, a drive in the country, indulging in that brownie, sitting with our favorite book, getting a massage, taking a bath ... we ALL have SOMETHING we do to make us feel a little better when we are in need. So, why do we expect our kids to NOT have something? Seems like a paci is a fairly benign commfort item. I would rather have my daughter use a paci to comfort herself then a piece of chocolate cake.
Seems to me like we often try to squeeze our kids into this adult box of what we think they should do. Obviously there are things where we do know best and they just need to obey. Not touching a hot stove, wiping after going toilet, not loading up on sugar, etc... But there are things where it seems like we expect them to be adults and understand adult things when ... they just aren't ready. I'm not an advocate of coddling your kids... but just letting them be kids.
So... just something to think about.