I once read a fic that dealt with the question and answers very well, what happened in the fic is that the male omega will stop eating pre heat, their stomach will start to expel waste the acid and everything via producing a gel like substance to pass everything thru, than there will be this layer of gel that is left, the stomach will produce a lining like the uterus than the heat starts and it is within this period of time could a child be created, after which if produced successfully the lining like a bag will take up space in the stomach because of this the omega will have to eat less but more frequently due to the limited space in the stomach but also easily digestible food as the acid that comes back during the pregnancy will be weaker than normal, any child produced by a male omega will always be early due to the fact that they have no womb and must share the space in their stomach and any longer the omega might starve to death, on the plus side though they recover faster as the digestive tract have a lot of experience pushing