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You need to contact an attorney or represent yourself and do a guardianship of the child. If his mother consents, it should not be a difficult process. You do not state anything in your question about the father, but it is likely he will need to consent as well, or you will have to make efforts to find and notify him.
If his mother won't consent due to the welfare issue, you can attempt to continue without her consent or work something out with her.
In most states "guardianship" is essentially equivalent to "custody"; the custodian/guardian makes decisions about and for the child, is responsible for the child, can authorize medical treatment, etc etc. Sometimes guardianship is given to a non-relative whereas custody is given to a biological parent. Although there are some minor differences, "guardianship" generally means the same thing as custody: a package which includes all the rights, duties and responsibilities of a parent. There are minor exceptions and variations here and there- in some states "custody" may not include authority over the child's estate (i.e. their property) whereas unqualified "guardianship" does. This kind of thing can vary depending upon the state the child resides in.
Hope this helps!