My son is 13.8 yrs. old, 141.1cm tall, weighs 41.8 kg. is it safe to give him vit. b 50 complex for increased appetite?

Yes, and no increase: B 50 is a very high dose of all the B vitamins, which are all water-soluble,; they all have a maximum amount which can actually pass from the intestines into the bloodstream every 8 hours. The rest goes to the large bowel and is eliminated. So, it is impossible to OD on them. I doubt, however, that there is any evidence-based data that giving extra B vitamins can stimulate appetite. Exercise!