For train choices consult the English version of
http://www.RENFE.es . For bus, the company covering this route is at
http://www.auto-res.net/ . On both sites you'll find schedules and prices.
For the bus option, your END POINT in Madrid is/should-be Conde Casal (METRO). From there you'll be able to get the metro (or a taxi) to the Puerta del Sol.
While not positive (consult other postings on this route here in this forum category - or someone can offer experience here), I think the Salamanca->Madrid train will end at the Atocha train station. From there your son will be able to get metro to the Puerta del Sol. From there he can walk to the Hostal Persal.
Saludos, MadridMan
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