Welcome to the wonderful world of the gifted, and it will only get more amazing and crazier, F. B.. (Love your screen name.) Anyhow, our house is bilingual too. (English/Chinese) The only difference is that we capitalized on English and figured he'd pick up the Chinese later. Well, DH and MIL waited too long to try to teach him Chinese. (I told them to start at 4 yro and they waited until 10!) He can't speak much Chinese at all, but his English vocab is amazing so being bilingual doesn't mean a kid will have touble with learning in school like some teachers think. Anyhow; my DS was like that yours too with learning new things. The only difference is that he was used as a peer tutor/example to help children with speach problems. My DS came home one day and verbally spelled his name when he was almost 2 yro. When I asked him how he knew, he told me he read it off his cubby at daycare. By 2 1/2 he was already printing his name forwards and in mirror writing. (He told me he was bored writing it the normal way and wanted to be different.) Your son is probably picking things up through observation. Knowing the sounds of the letters could be coming from anything like knowing the names of the children in his class and seeing their names, then associating the sounds of their names and the letters in the names. If you or a daycare teacher reads a story and he follows along, that could be another way he could be picking it up. In the grocery store, you probably say the name of the item and he can see the label. ("Billy, do you think we should pick up some animal crackers today?") You might want to capitolize on his strength and encourage him or have fun with it with him. Get the book, "Hi Pizza Man" by Virginia Walter. This book is great, because it's simple and repetitive making it easy to pick the words up. I know he'll love the book like my son did. He just couldn't get enough of it. Then read it out loud, but point to the words as you read. Sooner or later, he will start reading to you and let him do it. Other good books are:
1. Round is a Mooncake
2. Brown Bear, Brown Bear, What Do You See
3. Going to Sleep on the Farm by Lewison