My son wants to play a G2 droid in an upcoming game and I am trying to figure out how to best build it for him. How would you guys do it?

Sounds like a Technician to me, although one with a higher Presence/Cunning than you would usually expect from a repair droid.

Agreed.

Good Leading Spec options would be Mechanic and Outlaw tech, with Slicer and/or Rigger as possible secondary also the right side of the Scientist Tree. If you're going outside the box, Saboteur or Demolitionist might be interesting, as well as Thief, Analyst, and the right side of Infiltrator and Assassin might also come in handy.

Bump up Intelligence, probably followed by Cunning. Consider bumping up Agility, Willpower, and Presence To gain useful skills and increase your initiative. Brawn should be your lowest priority.

Gear will depend on what trees you take and your "cover" if you're not what you claim to be.

Assuming you're a technical character a toolkit is probably a good starting item. After that you can choose additional gear as appropriate (don't forget to take some repair patches since you can't use stimpacks to heal). Comlink, hand scanner, heavy clothing, scanner goggles, combat knife (used more as a tool then a weapon), and glowrod are all things to consider. Think about the movies and TV shows with a robot, and how every little dot and greeble have a function and factor that in. Also look closely at items that can remove setbacks or reduce difficulty for you and the rest of the party. Integrating a glowrod into your character is just a matter of buying the item and then asking the GM if your Eyes can now be flashlights (a common robot trope). He'll likely say yes, and boom, your character can now light up a small room and remove setbacks for darkness!

Also keep an eye out for items that have a lot of possible uses beyond the original intent, or can help you overcome critical weaknesses. An ascension gun is great for ignoring some Athletics checks, and it's also a handy way to yank far away items to you.

As your character will have a low Brawn don't forget to load up on Encumbrance Threshold increasing items early in the game. Backpacks, modular backpacks, load bearing gear, utility bests, tool vests, and so on will be critical to ensuring you can carry all the critical tools you'll need to pull out of your shiny metal butt. Be like a tiny robotic Batman.

Remember you can change the wording to "dress" some items as others more appropriate for your character, also useful when trying to convince your GM to let an item be integrated. Examples include: Backpack = Tool storage rack, Combat knife = utility saw, Bolos = Adhesive Sprayer, Heavy clothing = Standard Service plating, Gas Grenades = Space parasite fumigation canisters.

The 3 common Characteristics options players have for droids are: 441111, 432211 or 333211, for the enjoyment factor of rolling lots of dice early on i suggest the 432211. Then just choose which actual characteristics to allocate the score too. The 441111 option relies very heavily on skill ranks and is a bit minmax (although 522111 is worse!) so creates a lot more scenes where the PC is not able to contribute as well.

All droids rely heavily on skill ranks to improve the dice pools of the skills associated with their low characteristics, so to make them more well rounded focus on some skill training with the first few sessions of XP. They also should be making the most of the large Cybernetics limit they have, consider providing these as rewards as they are very expensive.

If you're going for a mechanic type your son needs to decide what kind of mechanic he wants to be, vehicles or weapons or equipment. Tell him about the new Tech book due out Q1 next year too, he may want to start in outlaw tech and pick up Droid tech or cyber tech later on.