Yorkies -- How old does my boy have to be to breed?

My little boy Yorkie, Tucker, who is almost 8months old has been following my female Yorkie, Presley, around for days. Everytime I look at him he’s trying to mount her and I say his name and he immediately stops and she jumps up into my lap. Neither of my two babies are fixed and I don’t plan to have her fixed until she does have a litter. I’m not going to sell the puppies if she does have them, they are just going to be an addition to our family. Both of them have went through eye and hip testing as well as both of their pedigrees are in my posession. They each came from completely different breeders and have no connection in anyway, my concern is if he the ability to get her pregnant so soon? He will be 8 months old on 7/11 and she will be 2 years old on 9/9. She has just finished her second heat I believe and I’m just concerned because normally she eats with Mommy when I eat (lol) and only then. But she is snacking so to speak throughout the day and if I don’t have food down or she runs out of water in her dish she immediately starts scratching the bowl, which has never happened before. Can anyone share some ideas or thoughts? Their vet said that it’s possible but not probable and it’s too early to tell with her.