I think Melissa has outlined the problem well. A new baby in the family is often the cause of potty problems - if an older child can be more like the new baby, he intuits that maybe he'll get some of the attention he has lost. He doesn't like the situation he has been put into by the adults and circumstances in his life, and body function is the only thing he feels he has any control over.
If pottying behavior doesn't get the results little siblings hope it will, occasionally kids will go on to act out in other ways, with eating disorders, breath-holding, tantrums, or other destructive behaviors. They are are too young and inexperienced to be effectively strategizing. They simply go with the first thing that gets a reaction from their parents.
So it's really going to be most healing for all family members if these poor, stressed parents slow down and interact with great patience and love toward their little guy. He's hurting, feeling lonely, abandoned, displaced, frustrated and perhaps even unwanted (his own emotional interpretation of events – I'm sure his mom still loves him dearly), and he's confused about what to do about it.
At his age, he simply doesn't have the perspective to see that he might have other choices that would give more positive results. This is the only snippet of power he's managed to discover.
This is where parenting has a chance to shine. If the mom can possibly find it in herself to spend more time snuggling, talking, asking the child about what he needs (this can be a slow revelation over many little talks, because he's just confused right now and thinks he's mad), she might find that life becomes happier for not only her son, but herself, too. Sometimes positive change starts right away.
