Farewell, thou child of my right hand, and joy;
 My sin was too much hope of thee, lov'd boy.
Seven years thou wert lent to me, and I thee pay,
 Exacted by thy fate, on the just day.
the fact that he used right hand could also show a bit of favoritism because the right means good often times where left indicates bad. This line is also written in iambic pentameter.  Johnson had a great love and joy for his son.
On My First Son