Do I need an additional legal document for guardianship to redraft a deed that was put in my son's (14 y.o.) name only?

Respectfully, you are confusing the various legal uses of the word "guardian". A "natural guardian" is any parent who has not had their rights disrupted by court action. A "guardianship" is a legal devise to transfer the powers of a parent to a non-parent. Certain guardianships may include the power to convey real property because the guardian acts as a fiduciary. I suggest you have a lawyer look over the documents conveying (gifting) the property to your son. It may very well contain a provision or restriction on the sale of the property, or how , if at all, it may be alienated from the child. The mere fact that you are the child's "natural guardian" may not be sufficient to grant you power to convey the property.

Typically a minor cannot own property in Massachusetts. In order to be given the power to manage the property of a minor one would need to be appointed as a Conservator of the minor's property. The Court would then be able to authorize you to sell the property on behalf of the minor. The Court would also then require that the Conservator manage the minor's property until they reach the age of 18. This can be a complex issue and one that you should consider hiring an attorney with experience with Conservatorships for minors.

You need to petition the Probate Court to be petitioned as your son's conservator (not guardian!). If the timeshare is considered to be real estate -- that is, the ownership is transferred through a deed -- you will need to petition for a license to sell the property. I strongly suggest you meet with an attorney who regularly handles conservatorship cases and get more assistance.