So now the boy is gonna ask how to relax that second sphincter. Dude, until you have enough experience with huge dicks, the best way is for him to lie on his back and you sit on him. You'll have far greater control / comfort and may be able to "go deep".
On a different note... and for the experienced men... I'm not a big fan of getting dicked pass the second sphincter. I find I often get sick (stomach bug kinda thing/diar.) after someone has gone "deep". Kinda thinking those first six/seven inches are tougher and more resistant than past that second sphincter.