How to properly bring remotely working freelancer to US for a short period of time?

We have our lead developer working for us remotely outside of USA.
We would like to bring him over to US for a month - what kind of visa should he get to do this properly? Is B1/B2 a right choice? But will he be able to work (do coding) during this month or its prohibited when traveling on business visa?

I am sorry but Karlson is completely incorrect. You cannot "work" at all in the US on a B-1/WB Visa.

The definition of business under immigration law is rather limited. Generally speaking, a B-1 visa holder is not permitted to work, receive payment from a US source (other than reimbursement for certain expenses), or run a business.

A B-1 visa holder may:

• Negotiate contracts Consult with business partners
• Attend trade shows
• Participate in scientific or business conventions
• Solve litigation related issues
• Interview candidates
• Hold meetings with suppliers and customers
• Showcase samples and solicit sales

You can only "work" on a J-1 Visa, H1b Visa or B-1 in lieu of H1b Visa A J-1 Exchange Visitor Program Visa will allow a foreign national to work, train, or travel legally whilst experiencing life in the United States. This is the easiest visa to get and will usually take 2-8 weeks to get but the hiring company will require 5 or more employees and will have to have been a running company for 12 or more months to qualify at a minimum. The US company can only employee for a maximum of 18 months and usually only hire that employee only once undert he J-1. For more information, click here.

An H1b visa is a non-immigrant visa that allows US companies to employ foreign workers in specialty occupations that require theoretical or technical expertise in specialized fields such as in architecture, engineering, mathematics, science, and medicine. Under the visa a US company can employ a foreign worker for up to six years. This visa is much harder to get and is limited to how many are given out a year. The visa can only be given out after October each year until the cap for that year.

For a B-1 in lieu of H1b Visa, in certain, limited circumstances the US Consulate may issue an employment-authorized B1 visa where the work to be undertaken would usually require an H1B visa. This provision is particularly applicable to situations where you may need a non-US company to send a member of staff to the US for a limited period in order to undertake specific projects for you, or where you wish to bring in an employee of an overseas subsidiary, affiliate or parent for a limited period. The requirements for acquiring a B1 in lieu of H1B are:-

• The work to be undertaken in the US must be H1B level – i.e. the worker must be engaged in a 'speciality occupation';
• The worker must permanently employed (i.e. not a contractor) and paid by the employer outside the US;
• The worker may receive no compensation other than expenses from a US source;
• The worker must have a degree relevant to the services to be provided– there is no provision for work experience to be considered equivalent to adegree, as there is under the H1B.

The B1 in lieu of H-1 visa generally takes 1 to 2 weeks to obtain, and considerably more supporting documentation is required than for a normal B1 visa. Periods of admission and extension are the same as for the standard B1 visa (i.e. generally 6 months). For more information, search Google for the term "B-1 in lieu of H1b" and then the country you are from after the term.

These are the main visas you would be looking at but there are many more.
Click here for more information on all US visas.

Hope this helps.

Depending on what country he is a citizen of the developer will need a B1 or a WB visa. You can take a look at individual links to gather the requirements and see the necessary forms.