Current Social Solutions
Education
To stop the growth of the population and cities there can be a more in depth sex education system in place in the school systems. These teaching will help people understand at a young age that they should think before they start having sex. Planned parenthood should also be explained to the general population so that the point is further enforced that we as a population need to be wary of the consequences of being reckless with reproducing.
http://www.plannedparenthood.org/
http://www.plannedparenthood.org/
Government
The individual governments of the world can offer tax incentives to families that have only 2 children or less. This incentive will give people more reasons to be careful with their reproduction and keeps the overall population manageable. There could also be penalties for having more children, this may seem extreme but this will only benefit the good of mankind so that humans will not have to worry about terrible living conditions and running out of resources. Readily available birth control to the general population will help maintain a stable population. Some religious groups will be opposed to this however it is not mandatory for them to accept the birth control and if they see what the world will be if the world exceeds carrying capacity some of them may change their mind.
http://geography.about.com/od/populationgeography/a/onechild.htm
http://geography.about.com/od/populationgeography/a/onechild.htm