When her own dear boy is far away.”
Line 33: "“If ever she’s poor and old and grey,"
Line 34: "When her own dear boy is far away.”"
" If ever she’s poor and old and grey, "
(He imagines that his own mother may one day become old, poor and grey.)
" When her own dear boy is far away. "
(He imagines being absent when his mother needs help.)
Overall meaning: He imagines a future when his mother may be old and he may be far away.