Hi!
Any operation is going to cause trepidation. Thankfully operation practice and techniques as well as recovery care have made huge strides in the last years and the majority of operations in small animals come off well and without a major hitch. Don't be thrown by all the horror stories you can find online. People are very keen to share a bad experience, but won't do the same for a success story so the picture is always very skewed. The same goes for our health/illness section - we get contacted over problems, not over things going without a hitch!

In your case, a neutering operation is indeed very much incicated to prevent long term complications.
Here is information on neutering ops and post-op care, which you will hopefully find helpful:
Neutered / De-sexed Boars And Neutering Operations: Myths And Facts (includes a link to undescended testicles information)
Make sure that you have the necessary support care at home (feeding syringes, recovery formula etc.) so you can step in asap if necessary. Hopefully it won't come to that - but being prepared is always best!
Tips For Post-operative Care
I had a 6 months old neutered on 2017. You can find Nye's story in the neutering guide. He is still around, still together with his mate, now 4 year old Nosgan and doing fine. My new baby boy Llelo (recently paired up with 2 year old Dylan), who was one of the 3 boys born here at the end of August during an emergency holiday piggy sit for a new owner with a highly pregnant sow is going to be neutered as soon as my vet is prepared to operate on him- which is a lower weight than your boy already is, by the way.
It is more important that your boy is fit and healthy in himself and is on a good hay based diet. He is big enough for an operation and young enough to heal quickly. Fat, especially yellow fat around the organs from overweight, is an operation risk and not a bonus.
Long Term Balanced General And Special Needs Guinea Pig Diets
The Importance Of Weighing - Ideal Weight / Overweight / Underweight