Most children (unless they have a brain injury or other condition that may limit their ability) will become quite intelligent, as long as parents don't make them too unhappy, frustrated, or generally frazzled to be interested in learning.
Where it can go wrong is when parents think that once their child knows the alphabet and a few numbers that's all they need to do as far as learning goes. It's not. They need to encourage an interest in learning as long as the child is a child, and one way to do that is to be an adult who is interested in learning and reading, as well. It sets a good example because children emulate parents. I've known a lot of kids, including kids from single, teen, mothers and they all seem to start out awfully, awfully, intelligent just by being loved and getting attention. It's when many kids start preschool or kindergarten that they start to slip back because they haven't been properly prepared for learning beyond the first few years.
Children who are secure and happy are good learners, and being someone who learns quickly and easily is what intelligence is all about.
Meet his needs from the day he's born. Make him feel secure. Treat him with respect (and expect it back when he's old enough). Let him know you enjoying being with him, and talk to him a lot. Also, make him laugh frequently by being silly. Introduce him to all kinds of things in the world that may interest him.
Where does my opinion/answer come from?
My own three (now grown kids) were all happy and intelligent. A couple of them read at 11th-grade level in third grade. One had a psychological evaluation for school and was said to have an "amazingly positive view of the world". They're multi-talented people with lots of common sense.
It didn't take any special efforts - just making them feel secure and happy, having a positive relationship with them, and letting their natural abilities blossom (rather than "suffocating it" with a lot of negative interaction between us or around the children) (and whatever else I said above). As I said, I've seen lots of other children equally intelligent and happy to my own. That's because most children are born to be happy and intelligent - but adults mess them up, sometimes only in subtle ways, by the time they're four or five. They may still have the same potential, but without interest in learning they will lose their advantage within a very few school years.