I've seen it work both ways. Parents I know who put their kids in good daycares, pre schools, Pre K and then kindergarten and the kids loved all of the above and of course progressed nicely in all the steps, and I have lots of relatives who home schooled and didn't send the kids anywhere before 5 year old kindergarten and just took them to parks etc to be social. This also worked well and the kids adjusted fine and had all the same social and school skills form parents being diligent at home.
If you're a teacher and working with him at home, and you don't see any signs of alarming shyness when you're out with him, you can really take your pick based on budget and time. For my first, I worked PT and put her in daycare 2 days per week when she was 3, then I put her in K4 (the daycare would have kept her until age 5, but why when she could start the structured classes etc. She loves it and it has definitely been great prep for kindergarten, it will be no adjustment at all. Meanwhile, my youngest two are home with me and I haven't even thought of putting them anywhere, because we don't have the budget for more than one kid enrolling in stuff :-0.
My 3 year old son is great with his letters etc, and I teach him piano and talk to him A LOT. He's very comfortable in social settings, so who knows if I'll get around to preK for him. He sort of learns form the homework his sister brings home. :)