Is your boxer a front or hind leg amputee?  My husband and I have been through several surgeries with fur kids to correct ACL issues in the knee.  Our first pup Lucky had both his knees done (5yrs and 9yrs old).  Our Leland had gone in for a TPLO procedure to fix his left knee which ultimately failed leading us to do the amputation.  He came down with a serious infection that the hospital couldn't get under control and the screws would not hold in his meniscus.  Then his remaining hind leg went out 4 weeks after amputation and his health was rapidly declining which led us to letting our boy go instead of putting him through more surgeries that in all likely hood wouldn't work.

If your boxer is a front amp then I would suspect recovery from ACL surgery may be a little easier.  There are several different procedures now to fix ACL issues but the recovery period is quite long before you can let your pup put full weight back on the fixed knee.  I want to say the typical recovery time is around 12 weeks that you'll need to assist your pup with a belly sling and having them go on leashed walks to potty.

It is doable and like Jerry mentioned I would seek out a 2nd opinion from an orthopedic surgeon.  If your pup still has a will to live I wouldn't count out surgery just yet.

Best Wishes

Sahana and her Angel Leland and Lucian too