Breeding mother to son will not cause any problems unless both of them carry a recessive gene for a problem- and that is impossible to tell until you see the litter. Linebreeding when done by someone with a handful of genetic knowledge and knowledge of the bloodline he or she is using, is a great method for multiplying desirable traits. You don't say what you are breeding for but James Dilley's advice is very good for breeding meat pigs or fair pigs. You can buy semen and AI your sows, relieving you of the year round cost of a boar and giving you access to quality boars without ever having to line breed. AI'ing sows is simple and straightforward.