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

I feel the question asked in the title and the issue described in the body are quite different, so I'll address them both, one at a time.

The English Problem

As you've described, your son is asking questions in an ambiguous way, and there is no point in teaching him the concepts behind one of the answers until he can clearly identify when that concept applies.

I see 3 occasions in your description where this was the problem:

At first, it was unclear whether he was asking about 4+2 or 42.

Eventually it seemed to be that he was asking about what the two numbers written side by side meant, so that's how I started answering him.

I did my best on the spur of the moment to explain place values but my explanation was lacking

At all of these points I wouldn't recommend answering the question until you have established clearly what it is he is trying to achieve. Instead, put the question back to him:

Hmmm, I'm not sure what you mean by "What does four and two make?" Do you mean a four and a two added together or do you mean a four and a two side by side? Because the answer will be different depending upon which question you are asking.

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.

You can also use this as a chance to point out that if he does get an answer that he didn't expect then maybe the person answering didn't understand the question and he might want to double check with them.

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):

Four and a two added together is six, because I had four and I put another two in as well. Four and a two side by side however, is forty-two, which is a lot more than six.

Once we've established this clear difference we have set the ground work for what you are hoping to teach him: why is 42 a lot more than 6!?

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.

At 4.5 I would also avoid the term 'place value', instead talk about position and keep it between 0 and 20 to start with.

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'.

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.

Failing that just plunk him in front of this YouTube video I made some years ago on the very subject! Sure, it's intended for adults and is a precursor to my follow up video on hexadecimal and binary numbers, but who wouldn't want their kids to understand those?