There is an agency which only provides male childcarers - My Big Buddy but there are some things I do not like about this agency from reading their website - their Big Buddies are 20-30 years old... so it excludes mannys like me (I'm 32). Not sure why they are excluding the older manny!
They are also London based, so may only deal with that area - though with luck they will expand in the future.
In your situation, your son has known your current nanny almost all of his life - so why change nanny? Instead, direct your nanny to do things that your son likes, such as muddy walks, train rides perhaps, playgrounds (especially the more adventurous types).
If your nanny won't do these activities, then you will need to go down the disciplinary route, so that after sufficient warnings, you have reason to sack her. However, I would have thought it unlikely that your nanny would want to do down that route, so just ask her to do other activities.
This week the 2 year old boy I nanny (manny) has enjoyed the following outdoor activities:
Playing in playgrounds
Walking in Windsor Great Park (and getting covered in mud)
Visit to Bodiam Castle, especially climbing up the tower (very big spiral steps)
Cleaning the car (getting wet is part of the fun!)
Should add at this point, his 3 year old sister also liked all these activities equally as much - so gender isn't really the issue, it's more that some people like being outdoors, while others prefer being indoors.