8 year ago the ratio of age of father and his son was $7 : 4$
Let the father and son age be $7x$ and $4x$ respectively
Present age of father and son is $(7x + 8)$ and $(4x + 8)$ respectively
According to question,
$(7x + 8) + (4x + 8) = 126$
$11x + 16 = 126$
$x = 10$
Present age of father $= (7x + 8) = 78$ years and present age of son $= (4x + 8) = 48$ years
After $7$ years, father $= 78 + 7 = 85$ years and son $= 48 + 7 = 55$ years
After $7$ years the ratio of ages of father and son $= 85 : 55 = 17 : 11$