my rat has changed his behaviour.

Question
Hi i got a pet rat (my first) in March last year, at first he was very shy and skittish however i soon over came this and he became very cuddly, friendly and playful with me. i was able to walk around with him climbing all over me quite happily. He use to love being picked up and stroked and all the fuss he got. In August i went away for 2 weeks and as he was very sociable with me i decided to leave him with my friend who has owned animals before, however once i got him back he had changed in temperament, and now i am unable to pick him up as he will bite me resulting in a lot of pain and blood!! luckily he is name responsive so i can let him out the cage and call him back into it, however i am very weary of him biting me so can not pick him up and hold him. He still likes to be stroked but its on his terms and i usually have to call his name before attempting to stroke him, when he wants to be stroked he will sit for ages to be stroked. He will run over to see me but it is on his terms and still will not let me pick him up. As you can see his temperament is quite dominant and the opposite to how he was. Is there any solution or suggestions for us both to gain each others trust. Or any ideas on the possible cause of this situation. Thanks xx

Answer
First, how old was he, and can you ascertain that this was a random behavior change? Or did your pet sitter note this behavior change? It could be a number of things. I would rule out a health problem first by having an exam done. If they determined nothing was wrong, if he is about puberty age (between 6 and 10 months), I'd have him neutered. Quite honestly, thats the primary problem with male rats displaying like that -- they simply think they're hot stuff. Neuter them and that behavior goes away and they go back to being their sweet selves.