As much as it pains me to have to go into this there are several medical and psuedo-medical options availible albeit most are somewhat questionble in their success and others have a unnervingly high chance of damage but heres a few;
1] a 'Penis pump' - does what is says on the tin its a vaccume chamber which is put over the gentlemans sausage and is said to be helpful for people with thin willys, its more for ED but never the less.
2] excersises - these generally include masturbation albeit stretching and directing the penis to try and make it longer or straighten it 'jelqing'.. weights are also common to pull it down.
3] Traction - Traction is a nonsurgical method to lengthen the penis is by employing devices that pull at the glans of the penis for extended periods of time.
4] surgery - width - involves injecting fat and is shown to hve short term success but more often than not long term results are unpleasant
length - usually involves cutting the ligament and a skin graft and might give someone 2cm however no evidence suggests it effects the erect one
5] liposuction round the pubic bone can make it look bigger.
1] Penis clamp, heavily unadvised but can work pparently, it involves rings/string etc. to constrict blood flow however can cause catastrophic damage and frequently lands people in the hospital.
2] pills and supplements acquired online - most will simply be sugar pills however others can be loaded with anything from hormones to god knows what.. doctors may prescribe there own or herbal ones can be used safely but most evidence suggests limited to no effect.
Heres a quote by Prof Wylie on them;
These products usually contain vitamins, minerals, herbs or hormones that claim to enlarge the penis. Despite their impressive claims, there's absolutely no clinical evidence that these products work and some may even be harmful. The University of Maryland in the US carried out an analysis on some of these and found traces of lead, pesticides, E. coli bacteria and animal faeces.
Basically the evidence says dont do it, low success rate massive complication rate and yours doesnt sound obscenely small anyway. Either way have a word with your GP for more sound advice and options.
And actually passed?