Explaining loss of pet to toddler

I am looking for tips on explaining the loss of a pet to a toddler.

We very suddenly had to put our almost 11 year old Golden Retriever.

Our 2.5 year old son adored him, they were best buddies who did everything together.

Only a few hours after we had to put down our pup, our son started noticing the his friend wasn’t around anymore. Our son is very compassionate toward animals in general, especially his own pets. He is also a very detail oriented child and is a “processor”. He has been asking where his friend went...that he wants to pet him...and keeps trying to find him so he can give him treats....

What are good ways to explain this to him? We want him to understand that his friend cannot come back...we don’t want to just say arbitrary things like he “went to sleep”...we want him to associate positive things and have good memories and not feel afraid...but also want to be honest.

Also how do we help him grieve? What are age appropriate things we can say? It’s easy for us to think he will just move on because he is so little...but really, truly he and our dog had a special relationship. He is going to miss him and look for him for some time going forward, I imagine.