There are a couple of possibilities here, but the most important thing is to be tough and brave, no biting situation ever got better because an owner acted afraid of an animal after a bite. Remember, you're the human, you're in charge.
You really need to look him over well. Sometimes if an animal who is usually tame bites it is because they are sick or in pain so check for symptoms of disease or signs of injury. If this is the reason he bit you the only cure is a trip to the vet.
If it's not disease or injury it could be because of the loss of Toto. First of all he really could be experiencing depression or anxiety due to being alone. Second he is male and sometimes males who are kept in a small enclosure or who are kept by themselves will become territorial. If this is the case you will notice that he bites when you reach in to grab him but then calms down completely once he's out of his cage. If it is either of these situations your reaction will determine the future. Remember he has no right to be territorial, it's all your territory! So don't be afraid, don't stop handling him or jerk back when he approaches you, if you need to wear gloves for a few days to feel comfortable that's fine, but be firm. If he bites again squeal, say "no" loudly and firmly, grasp him firmly and turn him over on his back. If this is an attitude or dominance issue then you will only make it worse by jerking away and putting him down when he bites you. This shows him that biting is an effective method for getting his way.