Depends if it's limit, or no-limit.
In limit, less than half a bet is a partial bet that can either be completed, or called. Half or more can be called, or raised [in this case to $150]. So his $50 bet doesn't actually count as a bet [usually there are 3-4 raises allowed, the $50 doesn't count as a raise or bet]
No limit, an opening bet is just an opening bet. The subsequent players can do whatever they want, but they must RAISE at least the BB. So in NL, you could call $50, raise to $150+ or fold.
In NL, an all in raise of less than half the required amount, can either be called or folded to, you cannot re-raise.
There is a lot of variation in this though. Most rooms have slightly different rules, and only about 75% of floor people will get it right in any particular room. You'll generally get a different rule in the morning than at night.