It's important to be honest with your boyfriend and tell him what happened. It may be uncomfortable and difficult, but hiding the truth could cause more problems in the long run. Explain what happened and that you tried to stop it, but your ex-boyfriend was too forceful. Be sincere and apologize for any pain this may cause him. It's possible that he may feel hurt, upset, or angry initially, but ultimately, honesty is the best policy in a relationship.
As for whether or not you should go see your boyfriend with the hickey, that is ultimately up to you. If you don't feel comfortable seeing him with the hickey, you could try covering it with makeup or wearing a scarf. However, if you do choose to see him, it's important to be upfront about the situation and explain what happened. Remember that communication and honesty are key in any relationship.