Hi Alex,
I agree with Gill - only resort to a gentle leader / halti type device if you can't teach Thomas to heel using praise, treats and a normal lead. Perhaps joining a local training class to provide you with help and advice - your vet or local grooming parlour should be able to help you locate a good training class, or even try your local paper for details on available classes.
I have to say that we used a gentle leader on Millie and were pleased with the results - however it irritated her face terribly - she was always stopping to try and scratch it off - even though we had had it fitted properly by our trainer and again by the vet. Now Millie has a kinda "habit" of scratching her face after having been out for a walk - even though we walk her on a normal collar and lead now.
HTH
Kel