As long as he remains an employee of the UK company, and paid by the UK company, the ESTA visa waiver approach will allow him to undertake a business trip of no more than 90 days. Anything beyond that will call for a business visa (probably B1) from the outset.
Consider also the need for medical and third party liability insurance, because if anything goes seriously wrong the costs will quickly reach hundreds or thousands if not millions..... Talk to your insurer

In reply to Ray:

Once had to manage similar arrangements, years ago.

Even then, entry to USA was a bit of a nightmare. Even on the visa waiver if the US Immigration Gatekeeper didn't like the look of some of my colleagues (beards / stubble / long hair / failure to understand British humour etc etc) they'd get rudely and aggressively interrogated and end up eg with only permission to stay 30 days instead and doubtless with their card marked as regards any future encounters.

So, it was very important fully to brief colleagues about etiquette when dealing with such Goons.

Thank you both Ray and David, especially the real deal regarding the goons!

I have warned our man but he tells me he knows already as he's been there on 5 different business trips this year of varying lengths to the parent company, probably for about 3 months in total! Alarm bells??

Does the ESTA permit such frequency, as long as there is a decent break in between trips, or are we very much in Visa territory?

He will still be paid by the UK company.

Kind regards

Polly

In reply to Pol :

As far as I know, multiple entry as a non-employed-in-USA business visitor is in principle fine and eg can happen a lot when such as UK-based Project Managers etc need to be to-ing and fro-ing regularly.

Sometimes colleagues with a too-short entry stamp would leave the US for a brief recreational excursion and seek re-entry at the end of this, and usually they managed to get a longer stay approved, even as a result of nipping in and then out of Canada - BUT, no idea whether such dodges might still work, so best not to try them, except in emergencies, and then to have a convincing explanation for the Gatekeepers.

In reply to Pol :

Polly, the ESTA visa waiver is valid for two years. It is possible to enter/exit multiple times within the two year period, provided that each stay is limited to 90 days. Multiple periods of long stays within the two year period will probably generate questions as to whether the work undertaken is for the UK employer (permitted) or whether it is really for the US mother company (illegal). If it is deemed to be for the US companythen incarceration and expulsion is a near certainty - as is being prohibited to enter the US again for a period of 2 or more years....
Note also that multiple long stays can easily generate a US tax liability for your employee. If in doubt, seek professional advice about your specific case
Ray