When he entered,was he given any problems at the airport?How long was the visa valid for?Its a really tight situation,the problem is that he is an overstayer and so if you want to get married,you need permission from the home office!this were it becomes tricky,if you apply it will alert them(HO)that there is an overstayer living with you and it will trigger a deportation order against him and they will have no problems paying you guyz a visit.The chikwamba case law does work sometimes just depends on how your case is presented by a barrister,whats worse is you guyz aint married.His case will have to go to court,he can argue on article 8 that it breaches his human rights.Seek professional help.The was a zimbabwean girl on this forum who left the uk at her own expense,she and her british born boyfriend left for zimbabwe in november 08 to get married and applied for a spouse visa,it was granted and she was back before xmas,its faster to get married abroad and apply for a visa from outside uk.Given the situation in zim,it is very scary indeed.You need a kick @ss lawyer to argue your case if you choose to apply in country,If money is no object,then go for it.
Is he working?,if yes then he needs to quit,if he gets caught at work or his employer sends out his details to the UKBA/HOME OFFICE.It will
be a 10 year ban from entering uk or worse prison then deportation!It will affect his application in a huge way.The government is really putting a tight grip on enforcing immigration rules.He should act now because things are getting tougher and tougher for illegal immigrants.keep us informed.
goodluck
p.s how old is he?