Assuming there is no bladder problem, your son's behavior is pretty normal. If he's "never been very good at telling us when he needed to go nor going on his own," some developmental specialists would say he's never really been fully trained, and is only responding to external prompts. Which, if you're busier now with another child, you may not be remembering as often.
That leaves me wondering whether he truly has gotten the mind-body-emotion connection that will enable him to be truly trained (some kids, especially boys, are not really capable until 4 or later). If that's the case, reasoning won't really have any effect, except perhaps to make him more willing when he is ready.
Or, depending on your tone, could make him more anxious that he's not able now. And anxiety, resentment – any negative emotional feedback – will complicate his emotional scenery, distract him, increase tension, and make successful training less likely.
I'd want to look at the family dynamics closely – be sure your son is getting appreciated many times a day for as many little things as possible, such as getting ready for his bath, putting together a puzzle, building a tall block tower, picking up a project when he's done, taking silverware to the table, helping entertain his little brother, lighting up the room with a big smile. The more he's noticed, the more his positive behavior is acknowledged (and this doesn't mean out-of-proportion praise, just positive notice), the more confident he'll feel in his new role as big brother. And of course, he'll thrive on regular one-on-one time with mom and dad, too.
I'd like to pass on a strong recommendation from other young families that I've known that can have a significant impact on both your sons' coming years; Siblings Without Rivalry by Faber and Mazlish. Though I have no little "siblings" in my life and haven't read that book, another by the same authors called How to Talk So Kids Will Listen, and Listen So Kids Will Talk is a brilliant resource, offering exceedingly effective tips that increase the mutual respect and cooperation in any family.
