I'm jumping into this debate several pages in, but I just couldn't resist.
Very few Welsh-speaking children have problems speaking English - I have never met even one. And in the OP's case, presumably, as an English speaker she will be speaking to her son in English at home anyway.
I fail to see why Welsh speakers should not have the right to education in their own language - and English speaking Welsh people are hardly lacking for schools!
To those who consider it a waste of time learning a minority language. There are thousands of languages spoken in the world today, a large number of which are minority tongues. Do their speakers not merit attention? Do they all have to be Nobel prize winners before they can be considered worth speaking to?
Incidentally, gaelic, words for modern technology are often borrowed across languages. I am a fluent speaker of Greek, a language that I assure you is very much alive and well, and words like "modem", "router", etc, are all taken directly from English. This doesn't mean that the Greeks are all really speaking English and playing at being Greek speakers, it simply means that Greek, like Welsh, borrows words. English does the same thing. Where do you think the word "problem" comes from? Or "paradise"? Or "flannel"? The list is simply endless.
My children are bilingual (English/ Greek). I consider it a positive thing. If we lived in Wales, I would send them to Welsh medium so they could gain another language, another perspective on the world.
Anecdotally, and may be irrelevant in the greater scheme of bilingualism, but I've noticed that my children have easily picked up a bit of Russian from a Russian speaker I know, where their monolingual friends have been slower. Even my SN son waves goodbye when he hears "da sfidanya!"