Alaric is right; virtually all of the aftermarket "upgrade" lights are in fact a serious
downgrade in terms of safety and legality. You're going to want to exercise parental authority and say N-O on this; he either needs to stay with the stock lamps (with correct, specified-type bulbs) or install
legitimate alternative taillights such as the factory export-type units with amber (not clear) turn signal compartments. Those linked auctions appear to be for appropriate lamps, but they also appear to not include the bulb sockets. This requires a lot of care and caution, because there are many sockets that will physically fit and click into place, but they are not all functionally the same...they hold the bulb at a variety of different positions not only rotationally but also axially, that is the length distance from the back of the socket to the filament. The filaments have to be positioned correctly for the reflector and lens, or else you don't get an appropriate, safe amount and distribution of light even though the lamp appears to light up.
Most trailer converters are intended to go the other direction (a trailer with combination stop-turn lights pulled by a vehicle with separate stop lights and turn signals), and most of them are half-baked and unreliable; they do a partway job for light-duty, occasional usage, but many of them can't hold up to day-in/day-out usage, and many of them create operational problems of their own...such as causing the rear turn signals to alternate with the front ones when the driver is stepping on the brake, and making random problems when the hazard flashers are in use. You can get a purpose-made control module out of Europe for what you want to do, like
this one or
this one. (all of Europe, along with pretty much the entire rest of the world, requires amber rear turn signals, not just some countries here and there, hence why there's a market for modules like this...to cater for the private importation of US cars). There is also
this US-made module, which is a lot less expensive than the European modules. I have no experience with it, so I can't vouch for it. On one hand, the maker has a reputation for catering to the "car lights are fashion accessories" crowd. On the other hand, they didn't set off any alarm bells or red flags in response to my fairly detailed questions about the function of this module. Maybe the strategy would be to buy it, give it a thorough test in all conditions (both turn directions and the hazard flashers, with and without the taillights; with and without the brake lights), and if it fails to function correctly under any condition, send it back.
Also keep in mind that while installing the export tail lights gets you amber, separate turn signals, which is good for safety (see
exposé), it also deletes the side marker light and reflector functions, which are required in the US and Canada and which are effective crash-prevention devices. There is probably enough wrap-around visibility of the tail light to provide reasonably adequate side visibility of the rear of the truck, even though it won't extend as far forward as with proper side marker lights. You can make a judgment call on that, but the side reflector function definitely needs to be provided. It's not hard; you can just apply
these (don't try to make do with just "any old" red reflector). Or you can install integral side marker light/reflector units, such as
these.
Sonny boy also should pay some attention to the headlamps on that truck, and pop should see that he does so carefully, appropriately, and in a manner that will (actually, really) improve safety rather than worsening it. That means no aftermarket headlight-shaped toys, no "LED bulbs", no "HID kits", no blue bulbs, etc.
