It is not true that fathers never win. The focus of the judge will not be on the mother and father, but on the children, because the primary focus must be on what is in the child's best interest. Normally, when the children have the same father and mother the judge likes to keep them together, and the same is still true even when the children have different fathers. Just present all of your arguments in favor of your son having custody. Emphasize everything he does on behalf of his child. And if you want to increase your chances of getting a positive outcome for your son, then have him consult with an attorney.
This answer is designed to provide general information only, does not constitute legal advice and does not establish an attorney/client relationship. I am an attorney licensed in Maryland and California. A consultation and retainer will be required if you would like to obtain my representation. Office: (410) 381-1656. David Mahood, Esq.
Although I'm in California, and things maybe different, below are a few of my general thoughts, obviously without there being an attorney client relationship, and without knowing all the facts and circumstances surrounding your specific case.
First, you need to change the your mind set that "Father never wins". You must go into it with a more positive attitude. Often a client's body language screams defeat even before the hearing begins, and if the Judge thereby picks-up on that, that alone may tip the scale ever so slightly in the other side's favor.
Secondly, Judges usually consider all factors when determining custody, including the ideal concept of keeping siblings together for the sake of their relationship, both now and in the future. If however there is a large age difference, this concern may then be come less of a factor.
The following are some of the arguments I would make: (1) the newborn infant will not really benefit from having a sibling around for at least the first year of life; (2) the older son will usually not benefit from having to share attention with the newborn, jealousy issues etc... In fact it maybe detrimental for the older son to have to deal with a new born step brother/sister; (3) custody arrangements can always be modified and changed to best address one or both side's concerns as the children increase in age. Therefore you can ask for separate custody of the siblings at least for the time being, allowing the newborn's family to settle in, and then modify it accordingly when things change.
Good Luck! And remember to be positive.