I give my dog Nexguard that says to feed him every month. But can i give him another one 3 weeks later. Because i think it has worn off becuase he is straching more than usual. Thank you!

Starching won't worn it down. Only a few ways to worn off:
1: bath/ swim within 24-48 hours after new application.
2: body metabolism
If I am in your shoes, I would wait until 4 weeks.

They shouldn't be given ahead of schedule.

Get out a fine tooth comb and check him over for actual evidence of fleas. They can often be found on the belly or behind the ears.

If you have fleas, talk to your vet about treatment--most medicines are preventative--as well as how to clean your home.