Too early to train--but, he wants to!

We adopted a 5 yr old and her 1-yr old brother this year. The records indicate big sister was potty trained at 1 1/2 but go on to say there is no proof of this claim made by birthmom..... Our son is almost 23 months and the last few weeks have been horrible---every five mins he pulls on his diaper and demands it to be changed.....even when it is still dry. I had him checked by the doctor and there is no infection or problem down there. He strips nude if we do not pin his clothes on him or dress him in unremovable clothes.....and recently has taken up a habbit of getting a 'present' out of his diaper and chasing sissy around. We have a potty chair and he likes to sit on it---but, only if all of his clothes are off......he is clearly 'not getting it' the few times we have let him sit on his chair...... Should I actually start trying to train him? I really think he is too young but, today I have changed 15 diapers in 6 hours due to his demands to be changed......urg this is driving me crazy as well as dressing him in combat gear---and safety pinning everything back on---I worry he will master the safety pins and get hurt---and if he is ready I don't want to prevent him from learning...... any input is welcome.