If he doesn't clinch the nomination before the convention, this will just about for sure mean that he'll run as a third party candidate.
However the deadline to register as such looms in Texas, May 9th, with almost 80,000 signatures. North Carolina's deadline is June 9 with about 90,000 siggies needed. Illinois, Indiana, and New Mexico are late June with the majority of the rest after the convention in July and August, and about 5 in September.
With the Texas deadline in a couple of weeks, he'll have to hurry. But if he runs as a third party candidate , he'll no longer be trying to win but to make things really bad for the GOP.
We already know how childish he is.
Oh, and Cruz, Kasich and the GOP are not acting like children? Give me a break from your silliness!