Can I spend down my dad's money legally?

These are good things to ask your Medicaid counselor. There are things you can do and things you can't. For example, you can replace the worn tires on your father's car, but you can't buy him a Rolex. You can repair the roof of his house, but you can't repair the roof of your own. You can purchases a funeral plan. IOW, you can do things that are deemed necessary to maintain his life, house, and car. Your counselor is used to these questions and will probably give you a fact sheet for your state.

To qualify for Medicaid, do you mean? If you have the authority to act on his behalf, you can spend it on things that benefit him, such as a pre-paid burial policy, a new wardrobe, a delux wheelchair, etc. If you father has significant assets I urge you to consult an elder law attorney, to be certain you are doing the spending in the most advantageous (and of course legal) way.