Trek 3700 XC Hardtail - looking to convert to temp road bike Bike Description

Front suspension, knobby tires

Frame: Aluminum w/semi-integrated head tube, bi-oval down tube w/integrated gusset, rack &amp; fender mounts, disc compatible dropouts, replaceable derailleur hanger

Front Suspension: Front suspension SR Suntour XCT w/coil spring and preload, 80mm travel
Wheels: Formula alloy FM21 front hub, FM31 rear hub; Bontrager 550 36-hole rims w/brushed sidewalls

Shifters: Shimano EF51, 7 speed? Something like that.

Brakeset: Tektro linear pull brakes w/Shimano EF51 levers

19" frame- I believe. 


My son is in his first triathlon end of May. He is 12, 5'2 and 31" inseam. So we gather he needs a Med bike 19"+(50cm) but going through growth spurts... We would love to get him a new/used road bike, however it is a big expense, and he just grew two inches this month (and summer is coming with more spurts on the way). I don't want to invest in a bike that will be too small for him in June, so finding a way to make this work.


So, we are looking at the temporary. We all have mountain bikes, and was recommended to us, that we switch out the suspension with light weight, rigid forks, and also switch out our wheels/tires. It is not a road bike, but will get us through till our son gets to a more interchangeable height.

What size, or type of fork is recommended? Is there a difference in size? We can afford alloy, or aluminum for both wheels and forks, and looking to spend about $200 for the temp. situation. And then tires, what is recommended. Are there additional things that are missing in the componentry that would be needed to pull this together? Not as in brakes or gears, just bear bones. We are not looking to build a bike, just have a go between solution while my son is growing. 

Suggestions, appreciated!

Cheers

( Yes I got the inseam right. You should see all of his high waders. He is leggy. Enough that the rest of him will be catching up soon.)
If you want to use a mountain bike on the road, you put on slick tires (this is all I'd recommend doing for bang for your buck). 

I wouldn't recommend changing out the wheel sets, since this is pricey and doesn't give you a ton of gains unless you're really at the top. 

A rigid fork would be a bit lighter, but you do need to match it to the type of fork you have there (this would be a good thing to consult with your LBS on, given that you need to get the fork cut and what not). I don't think this is strictly necessary (weight isn't that much of a killer, and this is a bit pricey for a bike that the kid is going to outgrow in a few months) - just lock out the fork (if you can) and move on with your life. It isn't ideal, but it wont make or break you. 

Most importantly is to probably fit the kid as best as possible to the bike you have. 

Also, 31 inch inseam seems high for a 5'2" kid.