Thank you all for your responses and sorry for delay in answering.
I already have the geranium oil (rose geranium) oils recipy, but I guess this is for humans, not for cats. As for neem, my cat uses neem product (not the pure oil), specifically for cats and it doesn't avoid him catching hundreds of ticks (I'm not exxagerating...). Neem oil also seems to work for very short time in humans (against ticks) but so far, for fleas, it seems to be effective for my cat. I've never seen any test on rose geranium oil though to confirm the time of action of it. Also, it seems coconut oil works well as repellent, as well as almond oil, but again, who knows for how long... And if cats can stand them...
Still, I wonder if any product stays on his fur (if not in oil form) because he constantly licks his fur and lives outside 100%. Diatomaceous earth is another product that I could try to put on his fur.... I'm just a bit afraid for his (and my) lungs... So many alternatives...
So I am now going to try next two things: garlic plus yeast products for cats, recommended by a veterinarian. It seems both yeast and garlic repells ticks (garlic for sure repells ticks as I use it for my garden very successfully so far).
The problem with cats (and dogs) is that there's research saying garlic can be poisonous to them (dangerous) causing a few troubles (anemia being one). But the veterinarian said that if it's not pure garlic, but something mixed, should not be harmful. As I don't have many choices (colars are a "No-no" as he can die hung on a tree or so; Frontline seems too poisonous in my view and I still feel it doesn't work well), well, I'll have to give a try for these yeast + garlic tablets.
the yeast info is available in German in many forums of animals. Some give regular baking yeast found in regular shops... I had previous experience (myself) with vit B ingestion to avoid mosquito bites that were totally inneficient (I did the Vit. B ingestion for months before going to Indonesia, malarial area, it WORKED ZERO)... So I wonder if yeast (rich in vit B) will help my cat....
I also wonder if I add a little garlic + yeast + sulphur homeopathic, if the sulphur smell won't be stronger and help repelling ticks as we're having an 'epidemic' this year....
I'm also giving him neem powder everyday, but he still comes with ticks (this last week though he had less than 30, so far), so a bit better than the average of 100 a week... 
thanks again,
Selma