You would have to ask your insurance company whether you can insure your son's vehicle on your policy. Insurers dictate the terms of their policies, including who, and what, is insurable. Check with your agent or a company representative, and get the answer in writing.
If your son doesn't live with you (and he's not a full-time student living in the town where he goes to school), the answer will likely be "no" -- unless you're a joint owner of the vehicle.
The sales tax payment is required because adding you as a joint owner means your son is "selling" an interest in the vehicle to you. Any time you adding a name to the title of a vehicle, it is subject to sales tax on the full amount of any encumbrance (I'm assuming your son has a loan, which means there's a lien on the title, so the sales tax due would be six percent of the total lien amount). Had your son included you on the title when he purchased the vehicle originally, there would be no additional tax due; it's the after-the-fact addition that triggers the tax.
The PA Department of Revenue has published its position on this on its website: http://bit.ly/2OB1km
Of course, as with all of my online answers, my advice is limited by the brevity of your question and the facts provided. Additional information would be required to provide definitive legal advice, so this answer isn't intended to, and does not, create an attorney-client relationship.