There really aren’t many options for bloodline residency. There is facilitated nationality through ancestry, but that requires 1 year of continued, valid residence (where the residence is continually maintained while the citizenship application is being processed - that can take between 12 months or longer). So if he is the son or grandson of a spaniard and you have the birth certificates to establish the ancestral links then after one year of valid residency he can apply. This is the method I am currently embarking on.
His other option is one I’m barely familiar with but have considered doing in the event that my mother concludes her citizenship process under the law of Historical Memory, obtains her citizenship of origen, and I find myself in an “irregular situation” (i.e. living in Spain with no residency permit = illegally). This is known as arraigo familiar and I will post below the link (in Spanish) about this method of temporary residence with the ability to work (renewable each year).
http://extranjeros.empleo.gob.es/es/InformacionInteres/InformacionProcedimientos/Ciudadanosnocomunitarios/hoja037/index.html
Essentially if he is the child of a Spaniard of origen he can apply while he finds himself in the irregular situation. Among the required documents you will need to pull the long-form birth certificate from the civil registry in Spain where the parent was born; if you have a copy of it this greatly facilitates the process, especially if you know the Book and Page information where the certificate is catalogued. The process is free and they will mail the certificate to you anywhere in the world (again, for free). It’s best to pull a new one. Here is the link to get this document.
Note: the arraigo familiar is doable only through the Spanish parent of origen. Obtaining citizenship through ancestry can be done up to the Spanish grandparent of origen, where obviously the parent does/did not necessarily have the Spanish citizenship or was even born in Spain. It is odd that degrees of ancestry are implemented differently, but this is how it works.
https://sede.mjusticia.gob.es/eCertificados/CertificadoNacimiento.do?lang=es_es&idtramite=1215327470593&idpagina=1215197884559
That’s about it. The only other thing he could do is try to get a residency permit in the home country to work in Spain. This is obviously more difficult because you may need to have a work contract ready so the consulate issues the residency permit.