Yes, it's possible. Below is from wikipedia under Rape_of_males
It is thought among the public that a male must be aroused if he gets an erection or has an ogasm, and so that means that they are willing and enjoying any sexual activity. Roy J. Levin and Willy Van Berlo wrote in an article in the Journal of Clinical Forensic Medicine that slight genital stimulation or stress can create erections "even though no speciﬁc sexual stimulation is present." An erection does not mean that the men consent to sex. Males can get erections even in traumatic or painful sexual situations, and this does not indicate consent.
Much like female erectile response, male erectile response is involuntary, meaning that a man need not be aroused for his penis to become erect; mechanical stimulation is all that is necessary. Arousal and stimulation are distinct things. Stimulation is a physical response to a stimulus. Men can be physically stimulated without feeling aroused and thus causing an erection. Men can be scared and intimidated into an erection, especially if the person is older or an authority.