Hi Jen. I start by saying that I am a speech therapist who works with kids age birth through two, however, I do not know your child so everything is just general suggestions.
First of all, you can go to the following link: http://www.asha.org/public/speech/development/12.htm
There is a list of things a typically developing child may be doing between 1-2 years old. Remember that it is an entire year range. The link is on the website of the American Speech Language and Hearing Association (ASHA), which credentials speech therapists.
A very simple guideline is: 1-5 words by 12 months; 20-50 words by 18 months; 100 words and 2 word phrases by 24 months; 200+ words by 30 months.
If your son likes books and is able to point to the pictures, I would use that venue to work on expression. I would try looking at the pictures and just saying "I see...." and then slowly point to pictures and say what it is (dog...ball...house). Leave some time in between words like you're thinking. Leave an extra long pause and point to another picture without naming it. See if your son will join in...you may have to wait for a bit but try to avoid "what's this?" Model what you want him to do. If it seems he isn't making attempts at all, you can say the word then model just the first sound (ball....b..b..b). You can even make the "b" sound on the back of your hand and his so he can feel it as well. Don't force the issue so that he avoids books or talking...try to make it natural and fun.
The best sounds to start with are "b,p,m" since they are early developing sounds and are easy to see on your lips. I do a lot with bubbles in therapy. I work on "b" with bubbles and "p" with "pop".
Some videos I really like are "Signing Time". They have a Baby Signing Time (try your local library). It introduces some basic signs as well as hearing and seeing the words/objects. They may be helpful for you even if your child isn't yet interested.
18 months is still pretty young as long as he is saying some things and he could very well "kick in suddenly" as some have suggested. Do keep in mind though that the process takes a while to be evaluated and to get enrolled in an early intervention program. If you do decide to get an appt, you can always cancel if you see a lot of change. I think it is wise to just be aware early with him since your older son was delayed in speech. The whole idea of early intervention is to catch things early with kids at risk and help them to catch up with the other kids before school.
If you have more questions or need help finding resources, click on my name and send me a message. Good luck!
K.