You can grow bird seed, but I doubt he'll eat any of it. I have a bird feeder in my garden and there's quite a lot sprouting underneath it. Joe simply ignores it.
The clover, dandelions, plantain, etc you can get from the link I posted are far more to his taste.
You can buy radish seed from most garden centres and the big DIY shops with garden sections.
Given his weight problems, I wouldn't mess around with bird seed, I would get stuff you know will help him from that site.
Are you in the Republic of Ireland or in Northern Ireland? According to the site, you will need to email them for a quote for postage if you are in the republic (see here). You can pay in Euros as well as Pounds