Hi Janis
There's a number of things that could look like a blister under the tongue, ranging from an enlarged salivary gland which has become blocked and cystic through to some kind of growth there. You are right to worry and should take him along for a vet to check.
Cathy Wickenden BVet Med MRCVS
Barton Lodge Veterinary Centre
1 Midland Road, Hemel Hempstead, Hertfordshire, HP2 5BH
Tel: 01442 216048 24 hours
[
www.bartonlodgevets.co.uk]