Is the President breaking the law?

Congress passed a law. The ACA. The President signed the law. Now it is “The Law of the Land”.

But over and over, the President decides whether or not he wants to implement parts of the law.

Nobody seems to think this is unusual. But I think if any other President, of either party, decided to openly ignore a law, plus arbitrarily change it on a whim, there would be a shit storm of protest and criticism. But not for Barack Obama.

Or, maybe it is it legal for the Chief Executive, who is supposed to enforce legislative action, to simply pretend it never happened?

If so, I would love to be educated about which Article gives the President this power.