Yes, the answer is puppies of this age do display this behaviour, I personally would remove the toys he is humping and or give a vocal command to deter this behaviour.
It is especially common with certain toys/teddy of similar size.
No advice for the OP but B has always humped her daddys leg when it takes her fancy from being about 12 weeks old!
She knows 'off' and will stop once she has started but she just seem to fancy her dads legs
Needless to say she has never dared hump my spikey tree trunks!!
Well, mine had a go on his bed at 12 weeks , the day after letting him watch telly inappropriate for his age (I did cover his eyes, but too late!).
Didn't last long, beds made from polyester, and I suspect friction burns!