It is a very complex issue with lots of options, some better then others. It is difficult to address all the variables in an internet post. If you PM me digits, I would be happy to talk to you. I don't do the surgery, but send them to the local surgeons. You want somebody who does a lot of them.
Generally speaking surgery is a far better option then letting it heal, which it won't. Once blown an ACL is gone forever. It may get "better" for awhile, but it will get arthritic, and surgery will limit that. Little dogs can get by without surgery, even though most would still benefit from it.
As to which surgery, that depends on the dog and the surgeon. The best option is to go to a surgeon who has done a lot of all the options and let them decide which is best. There are + and - to each, and a lot depends on the dog and its confirmation. One is not better then another, they just address different issues. Just don't have somebody "repair" the ACL. It doesn't work in dogs (they are different then people), the biodynamics of the injury need to be addressed.
If at all possible do to somebody with endoscopy as well. Most surgeons just do the surgery, but miss hidden damage seen by scoping the knee. Addressing the ACL, but missing the meniscus damage does not make for the best final result.
The hardest thing about surgery, aside from paying for it, is committing to keeping the dog quiet post surgery (3-4 months?) and doing the PT. If you aren't/can't do the follow up, it can be a huge waste of money. Also be aware if you have on bad knee, you probably have two, and it is not unusual to blow the other one in 6 months. Ultimately ACL problems in dogs is an anatomy/confirmation issue, not a sports injury (even though he may have blown it running or jumping, it was was just the last straw.)
Hope this Readers Digest version helps some, that and DD is an idiot, but that is common knowledge.