For max experts, vets and crew number, only class him fighter pilot, and only do that classing at 120. If you class ace and/or sqd ldr, you'll lose crew, and thus vets and experts with this special neut sailor.

how about boosting him??
when should i boost him??? 

Boosting works no matter when you do it. For all sailors

When you do class the lvl 90 neutral sailor you automatic get 30% vets, 30% experts and 40% recruits.

No, there are exceptions, especially with the lvl 90 neut. Any SS support, and all non-scout pilots should only be classed to the initial level. For subs, the added repair and restore ability of not classing them all the way is more important than the easily capped air/sonar. They lose 13 growth in those areas between 1st and chief after all, and that durability boost pays dividends for subs at all levels.

And for fighter/bomber pilots, the vet loss means that classing to ace or squadron leader will cost you in true ability. For ace, factoring the 30% vets, crew growth, and ability growth, you'll have a bit under 100k less true ability than leaving as fighter. That's about 3% less ability. Not a huge deal, but not worth paying the points for unless you're really tight on weight.