Very stupid of him to tattoo her name onto his chest. Nobody should ever do that, incredibly short term thinking. So many people regret doing that and thought at the moment they were so madly in love, but then they break up and are with somebody new and are embarrassed to explain the tattoo.
I'm not sure what you are asking of us to help you with? Just letting off some steam and wanting to hear some opinions perhaps? As harsh as this sounds then, there is nothing more you can do about it unless you are that charming and alluring of a lady to make him forget what his current gfs name was. You have chosen to let go, you would not be justified in having a second chance. Particularly if they are suppose to have a child together, it would be selfish of you to get in between and ruin this unborn child's future with a fragmented parental relationship.
It seems you two were pretty close at one point, and its been only half a year at most since the break up? If that is the case, I'm sure it feels like forever and that you can't get over it, but in the end, it has only been a few months. For such relationships, people could very likely go years before getting over their ex-lover. Sheesh, it took me 3 years to get over one girl, and we never even dated! We were very close though. You shouldn't try so much to be committing or thinking about other guys, it just doesn't work like that unless you want to be called all sorts of nasty words from your peers. Trying to drown it out of your mind will be difficult, however, for now I and others would suggest for you to go out and do different sorts of activities. I'm not sure what your age is, but I'm going to assume you are at least a student, so focus hard on your studies, go do some volunteer work, work out your body a little or whatever activities you enjoyed doing that can help you focus. Good luck, you will move on if you try, just do your best not to regret, life is too short to wallow in our misery.