It is NEVER EVER EVER too late to neuter! Go over it with your vet. Hopefully you have a vet who is familiar with bullies and give gas (isoflurane) only and no anesthesia like propofol.
Can it help with the marking issue? Yes. Will it for sure? No. Part of marking is territorial, and showing who is boss. He is also maybe marking because he thinks he is alpha, but you must show that YOU are the alpha. (I suck at being alpha but I try really hard!)
There are several health benefits to neutering. One of the most important concerns the prostate gland, which under the influence of testosterone will gradually enlarge over the course of the dog’s life. In age, it is likely to become uncomfortable, possibly being large enough to interfere w/defecation. The prostate under the influence of testosterone is also predisposed to infection which is almost impossible to clear up without neutering. Neutering causes the prostate to shrink into insignificance thus preventing both prostatitis as well as the uncomfortable benign hyperplasia (enlargement) that occurs with aging. It is often erroneously held that neutering prevents prostate cancer but this is not true.
Other health benefits of neutering include the prevention of certain types of hernias and tumors of the testicles and anus. Excessive preputial discharge is also reduced by neutering.