Just posting this again because it answers a question posted later in the thread. Sometimes a retained testicle is just in the canal right above the scrotum; sometimes it is all the way up into the abdomen. It just depends where it is what is involved in its removal.Has the vet checked to see if they can locate the retained testicle? Sometimes it is palpable and therefore, no stomach surgery is required. I have heard of testicles being manipulated down because they could be felt but hadn't dropped. It takes months to do but if that can be done, then surgery would not be invasive to get them out and off.
If stomach surgery is needed, then get the gastropexy done while it's open.