Here are link that may be helpful:
Erections in young boys
DEAR MOM,do not worry ,100% of all boys in the world rub their penis,is totally normal,just trying to feel good,the body has nerves.....a boy cannot physically masturbate until age 11 ,sometimes 12 ,or even 13...this is a discharge of semen,which squirts out called ejaculation and is also done by all boys and is normal.Now many times a boy can become obsessed with handling his penis TOO MUCH...or masturbating WAY TO OFTEN this is caused by anxiety caused by negative massages about his body parts being nasty or evil,,,,so be sure to start at a very early age to tell him that his body is beautiful and acceptable.you see,kids just naturally like to be naked as much as possible.......many families give a child the respect to choose if they wish to sleep nude,or be nude in the privacy of home...that guards against anxiety,and makes them feel accepted and happy.
"A boy cannot physically masturbate until age 11..." Nonsense. If you mean that most boys do not ejaculate earlier than that, well, sure, but so what? A tiny lad (or lassie) can masturbate joyfully and powerfully, reaching a full orgasm without ejaculations. Some authorities (especially Kinsey) assert that prepubescent boys are more capable of multiple orgasms than older boys, perhaps because they don't have to manufacture new sperms to replace the ones just sent on their way. Preschoolers hump their mats during naptime; babies have been seen enjoying their diapers and apparently reaching orgasm; ultrasound scans have revealed fetuses both male and female rubbing on convenient nubs of their surroundings. The earlier they start the longer they have to enjoy themselves.
My Aut. Bro does it all the time its normal.