When getting a tongue piercing the main thing you have to do is reasearch. Find out why people get it done, the health risks, and also where is the best and safest place to get it done. I would highly reccomend siting down with him and doing reasearch on it and also talk to a professional piercer and a doctor who specializes in body modifcations.

I don't think people send enough time researching where they want to get it done. You want to make sure it is a professional place that has acreditdation such as Associaton of Professional Piercers. Then you know they are  following proper safety and health codes. If you see that the place is not kept well do not go there.

From what I have read, tongue piercing may wear down your teeth depending on where the piercing is located and you have to make sure that you keep it clean in order to prevent infection.

I am a avid supporter of piercings in moderation because sometimes in society the more body modfications you have the harder it is to get certain jobs and also there is still a negative opinon about it. Certain professions and carrers it is harder to get into if you have them. If he is interested in other body modfications I would get them in places that he can hide them for work or if  he doesn't want them later on he can remove it easily.

One down side might be:  who would want to kiss that?  But from a mother's perspective, perhaps that's an "up" side.

One down side is also most employers in a professional capacity do not like alot of piercings. I have also heard that if done incorrectly the piercing can split your tongue.

It does NOT split your tongue!

As long as he gets it professionally pierced and he cleans/cares for it properly, there is little risk of anything bad happening.

I know of no up sides to this procedure. As a high school principal, I saw too many kids developing infections (this area is one of the germiest in the body and difficult to keep clean during the healing process. It causes some people to develop short-term speech problems because they have to adjust the way they form certain sounds. It can interfere with braces (yours or the ones of a person you are kissing). And as they others have said, it can be a real turn off to potential employers. When you go to work for a company, you are in part a projection of that company. There aren't many, except perhaps the people who make and implant the tongue jewelry, who would see this as the image they want the public to have of their business.

I've written an article in response to this question:

Tongue-Piercing Pros and Cons

Experiences with having a pierced tongue, and reasons why or why not to do it. Describes social stigma attached to having tongue piercing (oral sex with a tongue piercing and kissing with a tongue piercing), how to pick a piercer, dangers of getting tongue pierced, and keeping a job with a tongue piercing. — continue reading...

I would go with those who warned him that his tongue may spilt as well as he might be a victim of many diseases according to any infected tools.

Another disadvantage is that the jewelry can damage your teeth. 

You need to know the purpose of piercing your tounge. Our tounge is one of the sensitive part of your body and there is a posibility that complications may occur after the process. As long as you can bare the pain that is inflicted on your tounge then go ahead. Make sure to go only to those legitimate tattoe shops that follow the medical procedure to aviod infections and diseases.

it surely does harm to his body. 

I would never hire someone with a tongue, lip or nose piercing. Same goes with some tatoos.

If the individual makes that kind of error in judgement, what kind of damage would he do with my equipment?