A lot of intelligence is genetic, so you can't do much about that.
Children are also at a massive advantage if they have parents who are themselves well educated and articulate. If your conversation consists of "I was like... and she was like... OMG", then your child will sound thick. However, if you yourself enjoy language and enjoy manipulating it, this will be a massive advantage to your child.
Otherwise:
Talk to him, all the time.
Read to him, morning, noon and night. There are endless good children's stories (and lots of bad ones, too - be selective). But you can also read fun poetry to very young children. They will often laugh at onomatopoeia and 'silly rhymes' even if they don't fully understand the content. Books need to be an integral part of life. We have literally thousands of books, because I would rather buy a book than buy pretty much anything else. I always told the DC when they were little that their book budget was unlimited, but I wasn't buying more toys.
Sing to him.
Introduce letters and numbers as part of ordinary life, not as a "school thing". If you go to visit a friend who lives at 123 Acacia Avenue, get him to help you find the road (ooh, look at all those 'a's in Acacia Avenue - and those are the sound 'a', not the letter "A"), and the number.
Eat meals together, and focus on conversation, not on what he is eating.
Take him out and about - shopping, the park, the library, playgroups.
Let him discover the world as his own pace - poking in puddles, building dams, playing with sand and mud and stones. Let him tell you what he's doing. Guide him, but don't tell him. Be excited with him. Don't worry if he gets wet and muddy. He is waterproof, and clothes can be washed.
Expose him to the widest possible range of people of all ages.
Involve him in what you are doing - counting, weighing, folding.
Don't send him to nursery.
Keep him away from screens.
Even if it's entirely genetic, all of the above will do no harm at all!