My son is taking 5th year of Spanish and would take AP exam. Is it okay to not take it in senior year?
Do you mean to say that he will take Spanish level 5 / AP as a junior?
If he completes all the Spanish his school offers, then yes it is fine. Maybe he could do post-AP spanish, or find a dual-enrollment course at a local community college. This of course depends on the requirements of the colleges he is looking at.
This is valid if your son wants to continue in Spanish. However, no college requires or recommends 5 years of FL. So if your son does not want to take senior year, that’s fine, so long as the course he is replacing Spanish with has a similar rigor.
It’s not that he doesn’t want to continue Spanish but there are other courses that interests him more though he doesn’t want colleges to assume that he is a quitter.
They won’t. In the first place, while your son will spend hours and hours fine-tuning his application, AO’s will spend 12-15 minutes reading it, at most. They have neither the time nor the desire to psychoanalyze the applicant. In the second place, if your son’s school offers post-AP Spanish, congratulations. However, that school is in the minority. Third, your son’s reasons for not continuing are valid. Coulee admissions is stressful enough without worrying about every little thing. Relax a little. 
Thanks for keeping our insanity in control. 