What can happen if he does not pay and what happens if he does not get out?
The LL can file for eviction. A court hearing would be held and if it is found that he hasn't paid and an eviction is ordered and he STILL doesn't vacate then the Sherifff would come and put him and his stuff out on the street. As part of the eviction the LL can get a judgement for the rent owed, court fees, attorney fees, and interest. Bank accounts and wages can be garnished. Your brother will also forfeit his deposit towards the debt.
Can they change the locks on his door?
No, that is a "self help" eviction and is illegal in FL.
Can they take his personal property?
No, that also would be a self help eviction and illegal. However, if he is legally evicted and put out by the Sheriff they put your stuff on the yard or side walk and will not protect it while you leave to get a truck or secure somewhere to move it. So if the neighbors and passers by take it (which is theft) you are at their mercy and catching the people who do it is very difficult.
What day would he have to pay?
He received a 3 day pay or quit notice. He has to pay immediately. The LL can file for eviction at anytime. That said if he can't pay many LL will hold of on eviction if you are definitely moving out.
"That's just my opinion, then again I might be wrong." Dennis Miller