Indeed, a judge could find that the home you would provide for your son better serves his best interests even if you lived a distance from the mother, but it won't be because the child chooses to live with you. Children don't have the right to choose, and parents who empower them to do so lose control. Work with a very adept experienced family law practitioner to put together the presentation that will convince the judge that you are better prepared to reliably meet the needs of your child (and distance will become a less important factor).
Best wishes for an outcome you can accept, and please remember to designate a best answer.
This answer is offered as a public service for general information only and may not be relied upon as legal advice.
Your son is not yet old enough to elect who he is going to live with so your being awarded custody will depend on a discussion of the other factors that you discussed (mom's mental instability, your better environment, etc.). You should consult with and retain an attorney though. Custody cases are the most difficult with the most at stake. Depending on the facts in your case, you could request a mental or psychological evaluation of the Mother (which usually will also mean you should undergo one as well just to show that you are not worried about your test results and tha tyou are willing to do whatever you are asking the Mother to do).
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