Hi, Pat. Well, this may not be a question of development, but of personality. This baby may not be a big laugher, but that doesn't mean he isn't happy. The important thing is that he's loved and nourished; I'm absolutely certain that you are doing that fantastically. So be confident that if he's not screaming and crying all the time, he's happy, but he's just a quiet baby. As long as he smiles, he's feeling it, but he may not even be ticklish! Some people are just not ticklish.
My son happened to have been only a little bit ticklish, so he would giggle a little when I tickled him, but not explosively like some other kids. In general, he was always a quieter child, observing everything, not always inclined to reach out and socialize or to get excited. There's really nothing wrong with that, so don't worry that your child has a quieter nature than his older brother.
Keep talking to him, smiling at him, singing to him, kissing him, tickling him, and keep giving him all the other sweet, loving attention you do. Just enjoy the quiet, non-ticklish kid that he is.
Peace,
Syl