#The crux of your issue is an English problem, not a Math problem!#

##The English Problem##

##The Math Problem## Of course you still want your son to know about place value of numbers, and I reckon once we've established a clear difference between addition and position this will be made easier. Try not to combine teaching about one with teaching about the other too much (for example, 10 + 10 is 20 because the 1s are in the tens position...). At this early stage this will just muddle things and make it harder to establish the foundation which will be built upon as his understanding develops.

The crux of your issue is an English problem, not a Math problem!

The English Problem

The Math Problem

Of course you still want your son to know about place value of numbers, and I reckon once we've established a clear difference between addition and position this will be made easier. Try not to combine teaching about one with teaching about the other too much (for example, 10 + 10 is 20 because the 1s are in the tens position...). At this early stage this will just muddle things and make it harder to establish the foundation which will be built upon as his understanding develops.

Then you can drive the different home, tell him the answer to both regardless of which one he clarifies he wanted (although if he does indicate a preference then start with that one):

Also make use of visual cues: write the numbers down from 0 to 9, one above the other, then when you get to 10 make a point of emphasizing that you've ran out of numbers so need to put a 1 in the next space (to the left) and then start from 0 again. We're talking about position after all, let's show it! Get all the way to 19 and then explain you since there is already a 1 there you now need to add 1 it to get 20 and start again. As he progresses you can ask him himself what he thinks happens at 99 and then explain the next position to him. Once he is comfortable with this I'd start talking about position and how much each position is worth, but I'd say you are a wee bit away from that yet so will hold off elaborating in this already lengthy answer.

Then you can drive the difference home, tell him the answer to both regardless of which one he clarifies he wanted (although if he does indicate a preference then start with that one):

Also make use of visual cues: write the numbers down from 0 to 9, one above the other, then when you get to 10 make a point of emphasizing that you've ran out of numbers so need to put a 1 in the next space (to the left) and then start from 0 again. We're talking about position after all, let's show it! Get all the way to 19 and then explain that since there is already a 1 there you now need to add 1 it to get 20 and start again. As he progresses you can ask him himself what he thinks happens at 99 and then explain the next position to him. Once he is comfortable with this I'd start talking about position and how much each position is worth, but I'd say you are a wee bit away from that yet so will hold off elaborating in this already lengthy answer.

Lot's of good things going on here: We indicate to him that his question is unclear even though we heard it fine, we give him options he can use to make it clearer (which we should be aiming to teach him to use in future), and we establish that if grandma is giving him a different answer it could be because she didn't understand the question.

Use physical cues, such as counting blocks suggested in Rory's answer, even better if they are the type you can stick together, so when counting them out you can show that even though it is 1 'block' it is still worth 10 'blocks'.

Lot's of good things going on here: We indicate to him that his question is unclear even though we heard it fine, we give him options he can use to make it clearer (which we'll be hoping he sees the benefit of and starts using in the future), and we establish that if grandma is giving him a different answer it could be because she didn't understand the question.

Use physical cues, such as counting blocks suggested in Rory's answer, even better if they are the type you can stick together, so when counting them out you can show that even though it is 1 'block' (by sticking 10 blocks together) it is still worth 10 'blocks'. This will help build the idea that a '1' can equal '10'.