My kids are 18 months apart, and I don't think the older one every truly grasped what was going on. She would kiss and hug my preggo belly, but would do it after I had the baby too lol, so it was more trained than any understanding that there was a baby in there. All you can do is keep saying Baby and point to belly, maybe read some books. Your son probably won't grasp that it's a sibling until you bring baby home, and then baby doesn't go away. Ever. lol. I wouldn't worry too much about it :)
Also maybe in a couple months put out some of the baby equipment, like swings and stuff, so older son can get used to it. My older one always wanted to climb in the baby stuff and was nice to get some of it out of her system before baby came, so she wasn't jealous he got all the 'new' stuff.
I can say that with them that close together I have rarely had any jealousy issues that some new siblings bring.
Also maybe put together a basket of 'new toys' that you will bring out only when nursing new baby so older child 1)doesn't feel left out 2) Isn't making all kinds of trouble while you are stuck nursing!
Congrats on your second baby...two close together is alot of work...but also alot of fun to watch them grow together!