Ask him directly if he wants to marry
His words to you (timing) and to his parents (undecided) do not match. Asking which is true gives you something real to weigh before choosing to wait or set a limit.
If you do this: He may confirm he wants to marry and explain the parents' comment, making waiting informed; or he may admit doubt, which changes what waiting means for you.
Cost: An uncomfortable talk, and the risk of hearing an answer you do not want.
You: Ask him in a quiet moment whether he wants to marry you, and why he told his parents it was undecided.
The exact first sentence“When you told your parents we haven't decided whether to marry, was that true for you?”