At your son's age, I wouldn't be using LM as a primary math program. It is more for babies. You can still use it, but I would forget about following the planned curriculum. Here is what we are doing with LM that seems to be working for us. My son is 2 and a half.
Everyday we have a single math lesson - it only takes about 5 minutes. First, I show him the LM preset for addition +1. Then we count out the same equations with mini m&ms like this. (You can use raisins or any other manipulative.)
Me: How many m&ms are on the table?
Him: Zero
Me: That's right! 0+1= (pause to see if he answers as I show him one m&m and put it on the table)
Him: One!
Me: Yes!
We do this up to ten. If he doesn't answer I just answer for him. If he doesn't answer a couple of times in a row I stop pausing and waiting for him to answer as I figure he needs to hear me do it a few more times to learn it. He has gotten pretty good at it. After we add up to ten I show him LM -1 subtraction. Then we subtract the m&ms in the same way, but we both eat them. When he has all the equations down I'll start showing them in random order. When he can answer them all in random order we will do +2 and -2 equations, etc.
I use m&ms because it makes it fun for him. They are mini and he is sharing them with me so really he is getting a very minimal amount of sugar from it (about 2.5 grams) and he thinks it is the best game ever.
Today at the park he told me, "There are only two slides here. If we want to have three we have to go to the store and buy another one."
Later he was having fun giving me basketballs and making addition problems out of it. So I am very happy that he is not only learning but having a great time with it as well.
The thing you don't want him to do is to get frustrated. This is why I suggest leaving the curriculum and just focusing on a single set of equations until he can answer them easily. At first I would recommend you not test in any way. Just demonstrate. After you think he might be getting it, try pausing to see if he'll answer and if he doesn't I always take that as a clue that he is not ready to. I hope this helps. Good luck!