I would wait until 6 months for the Motrin, if you can. Another mom posted about the effects of Tylenol and Motrin, but couldn't remember which one had which effects ~ Tylenol in excessive amounts can do damage to the liver, whereas Motrin *may* have an adverse effect on the kidneys.
With that being said, when my 16-year-old was 5.5 months old, he also had an ear infection, and his temperature actually went UP (to a dangerous level) after I gave him the Tylenol. After that, I gave him Tylenol for "boo boo's" (cuts, scrapes, etc.) and Motrin for when he was running a fever, and then it was "by the book" ~ every 8 hours. For my 2.5-year-old, he now gets Tylenol throughout the day, and the Motrin at night, so he can sleep longer, but it depends on what is causing the fever or problem. But we started him with the Tylenol only, first, too.
Hope this helps, and that your baby boy feels better soon! :)