In "Son," a parent speaks to their child and says that their own wellbeing depends entirely on his. If the son runs away, there will be no rest. If he goes hungry, there will be no sleep. If he carries pain in his heart, the speaker can't feel any good in their own life. Each promise is built the same way, with an "if" that turns the son's troubles into the speaker's own.
The closing lines pull this together. The speaker says their life "only serves" the son, as though being a parent is their whole purpose. The last line, "You, my son which to love me bears," adds a quieter note: the speaker takes comfort in the fact that the son loves them back.
The mood is tender but anxious. The short, steady lines feel like a quiet vow, and the repeated "no" makes the worry feel heavy, even though the love behind it is plain. The wording is a little unusual in places, which gives it a sincere, slightly old-fashioned sound.