He has no legal obligtion to support step children. That one is DOA. The mother and bio-Dad of those children are responsible for their support. If the mother is not getting support for them, she needs to be seeking it, unless bio-Dad is incarcerated, deceased, etc. (In the case of his demise, she potentially would be eligible for Social Security Administration survivor benefits. So, even that reason probably won't fly.)
His current spouse also is responsible for providing support for ALL of her children, whether from they are from a prior relationship or the current one. She just cannot sit on her duff and expect a man who barely makes minimum wage to provide ALL of the support of the household. However, at this income level, if the children truly are very young, the cost of child care could well equal what she might hope to earn at a minimum wage job.
Moreover, if the original order deviated somewhat from customary CS guidelines in your state, a judge could make an ongoing order conform. I suggest that you obtain full and complete information about his current earnings. If he has been with his current employer for awhile, he may have received some longevity increases, even if his job is relatively low level. In other words, he may no longer be a minimum wage earner, even though he may only be making $10.00 an hour.