An ultimatum may not work as its competitive in nature and may not get you the honest answer you deserve. It likely you let this go on too long already, perhaps out of fear of the outcome. But its time some serious talk come about. his retarded excuse of him not wanting to "share his feelings" is just an excuse and a load of crap. All men have feelings, just as all women do. If he really cannot share or talk about his then perhaps he is no marriage material and should be seeing a shrink on this issue.
I am not talking so much of marriage, but rather the feeling of wanting at long term committed relationship. At some point this needs to be answered or this will only get worst for you. My 12 year marriage to my wife has been good and I have no regrets. But both need to enter marriage with the same need for commitments.
How about telling him straight out. Tell him you love him (if you really do), and tell him you want to be with him. But your not getting younger and you are interested in a long term committed relationship without a persistent "escape clause". Tell him you want a committed relationship and your not getting younger and have grown less tollerence to waste more time.
Suggest a one-month separation, not a break up, just a separation for both of you to regroup your own feelings. Marriage is both a selfish and giving act. But first it has to start with the self. Go on a separate vacation or something. Perhaps head back to college or something. This separation period should be sufficient and long enough for both of you to gain a clear mind for each of you want.
This separation will also tell him you're serious. During this time perhaps a short phone conversation is OK but no reunions and need I say, no sex. When you get back together both of you will have a more clear mind on what you both want to do. If either of you are not clear after this, then you may want to rethink the relationship.
A break up of a 6 year decent relationship will be hard, if not impossible for either of you so I doubt a break up will come of this. But if you have had an on-agaiin, off-again relationship, this may apply a clock to the relationship. But ask yourself, you two have been doing this for six years, suffered a miscarriage together and who know what else. Have you much energy for a uncommitted continual relationship with him?
I cannot really speak for your situation, I don't know you enough. I do know that if you don't do something your feelings will only get worst and ultimately hurt your relationship. Perhaps you cannot really go on a separation or vacation. You may have to tinker with the plan, but don't play games. Go straight to the point with mission, and he should be clear on this. After 6 years, both of you well know each other by now. The dating period has been long over.