Not being able to pull foreskin back over the head. is this a serious issue?

Can be: Can predispose to infections, foul odors, (rare cancers), stds, and the like. Phimosis is usually attributed to fungal infection of end of foreskin. Can be stretched (see: http://www.Glanspro.Com/ -- good luck!), or better answer is with urology consult for improvement in situation. Can cause paraphimosis problem if meatus (/"head") gets caught out ("strangulated by foreskin) when erect.