Things to notice:
At first, because there are a lot, he wants there to be 100 (100 is a big number and it's really cool--you'd be amazed how many children guess 100 any time they are estimating anything)
Later, he is counting carefully, and he is touching each cube as he counts it so that he doesn't lose track
But, when he gets to the decade numbers (30 and 40), he gets stuck and needs to ask for the next decade number. This is pretty common too.
Young children, also, usually have some point up to which they are pretty accurate, and after which they tend to make a lot of mistakes.
Other things children do that I wish I'd made videos of:
Children can count accurately from one before they are able to start the counting sequence in the middle. A child who can count up to 20, can't necessarily tell you what comes after 11 unless they count up from 1 to get there
Children can accurately tell the next number in the counting sequence before they can accurately tell you the previous number in the counting sequence. A child who knows what number comes after 12 easily, may have a hard time telling you what number comes before 12.