Hi there:
If you haven't tried Frontline Plus, I would give that a shot to see if that will help control the fleas for a full 30 days and do remember to treat the outside of you home as well as your pet. Below I will add a weblink that will tell you about a pet safe product to treat your yard to help with flea issues. :) As far as applying the next size up..I really don't recommend it but yes, you can normally apply frontline twice per month without adverse reaction...every 3 weeks is recommended in active flea areas. BUT you always want to discuss this with your vet first as they know your baby inside and out best :)
Diatomaceous Earth - Food Grade-5-lb - Bag - Diatomaceous - Garden Harvest Sup
(to treat area for fleas)
Good luck to you and your baby :)
breeder/healthcare specialist
15+ years animal health care Hobby breeder of Olde English Mastiffs and owned by an English Bulldog