I have an investment property with only my name on the title deeds. I no longer work but my partner is paying the mortgage. Would my partner be able to claim all expenses related to the investment property in his tax return if his name was on the mortgage or does his name have to be on the title deeds to be eligible to claim?

It would be a start to have a partnership agreement between you , but ideally a true partnership would be evidenced by the investment of both "partners". I expect that the mortgage would be sufficient evidence for deduction of the interest Tax doesn't drive the business get it all sorted out legally and it is beyond challenge

The answer isn't that easy in your case. What is the agreement between the two of you? Is this a rental property or just a property you are holding in hopes of appreciating value? Are you treating this as a business? The IRS recognizes partnerships even if there is no written partnership agreement - but you really NEED to have one. If you're doing this as a partnership, then you need to be filing a partnership tax return and income and expenses are divided between the partners as they agree.