You can't trespass on his property to remove the tree. The power company will usually come and trim the tree at no charge if it is growing through the power lines. Try calling them first. If that doesn't work I would suggest you file a claim in small claims court asking for an injunction to have him remove the tree. He has created a hazard by trimming it drastically and not removing it completely, which, by the way, requires a permit from the county.
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Under Florida law, you may trim any branches or roots that intrude onto your property at your expense.
As long as you have someone who is competent to do the task, you are generally not liable if the tree dies. I think this would be particularly true if he has cut the tree back that far..
However, if your jurisdiction has a tree trimming ordinance, you should be sure that the person you hire to trim the tree is properly qualified, and that any necessary permits are obtained.