Flexible working for an architect: ask the expert
As you say, it's relatively rare to find part-time employment in architecture (though obviously some employers do offer it). There aren't any obvious reasons apart from the culture of the profession - the job's no more complicated to organise on a part-time basis than many others. I think it would be worth digging a little more into your employer's reasons for needing you to return to work on a full-time basis.
There's just a possibility that if you understood what's bugging them, you'd be able to suggest a way round the problem which allows you to work part-time. If your employer thinks they need someone at your level on a full-time basis because the work's there to do, is there a chance they might be persuaded to allow you to bring in a jobshare partner (two for the price of one, wonderful!!)? If so, you'd have to do all the legwork - finding someone in the right area with the right skill set and with the communication skills to make it work.
Alternatively, if there isn't as much work around as your employer would like, could you persuade them part-time work for you will reduce the company's expenses during what will be a rocky few years, while retaining a loyal employee who's already got strong relationships with customers, suppliers etc? If you can't persuade the employer to allow a job share or (for the next couple of years) part-time working, then is there any chance they'd agree to use you as a freelance and help you find other sources of freelance work? Obviously, you'd need to suss out in advance whether working as a freelance would provide you with enough paid hours of work and enough notice to arrange childcare.
If you can't negotiate an acceptable deal with your employer, then you might like to consider trying for posts in the public sector or not for profit sector - both are more receptive to the idea of "family friendly" working than many private sector organisations are. Although there's a job squeeze on in the public sector, it's huge enough for there to be some jobs on offer. I wouldn't recommend you try to launch your own practice in times this hard, particularly not when you want "family time" with your son. I'd suggest you try to stay within the profession if you can, rather than look for something new. Part-time professional work is always very hard to find. If you stay within architecture, you'll be able to benefit from your networks with past customers, bosses, professional training bodies etc; you lose out on all these potential sources of help in finding a new job if you move out.
Linda Whittern is Director of Careers Partnership (UK) , a careers consultancy for people at all stages of their careers.