It is not at all rude to speak Welsh to your son, in the
presence of English speakers, BUT in deference to
the latter, repeat what you have said in English.
This way, the lad assimilates both Welsh and English
simultaneously, and, hopefully, so will the others in your
company.
Noam Chomsky has shown that the L.A.D. (Language
Acquisition Device) is genetic, but only operates
during childhood. By 11+ it's all over. Finito. Kaput.
Give your boy the best chance by speaking both
English and Welsh to him. He will acquire both
languages naturally, and without effort.
Once the L.A.D. has gone (as in your husband's
case) the learning of a second language is a
tedious and unnatural affair - fluency is very
rarely achieved.