Neither adding your brother to the loan, nor adding him as a joint tenant on the deed, will cause a "change in ownership," which is the requirement for a reassessment of your property taxes.
However, once you put him on as a joint tenant, if he survives you, he will own the property outright after your death. So, if your goal is to leave the property to someone else, then you cannot add your brother to the deed.
There is no legal requirement that your brother be named on title for him to cosign on the loan. So, if the lender requires this in order for you to obtain the loan, then maybe you want to try a different lender.
Hope this helps.
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