Sorry for the delay! Gill's reply to my email (edited to remove DS1's name etc):
There is remarkably little written/known about what preterm babies need in terms of weaning. The current rule of thumb (put together last year [2009] by an expert group of nutritionists/dietitians) is that solid foods should probably be introduced between 5 and 7 months (actual age). However, even the members of this group admit this is really just a pooling of their ideas and experience, rather than anything based on clear research evidence (of which there is almost none) - and they don't say which babies should start nearer 5 months and which nearer 7! So, at the risk of repeating stuff you already know ...
The issues for pre-term babies are a) developmental readiness (including oral skills, such as chewing, and digestive ability), and b) the need for nutrition. In terms of developmental readiness, it seems that they may be ready to digest solid foods slightly earlier than their corrected age would suggest (although they will still be 'behind', compared with their actual age), which is useful information IF spoon feeding is planned, or thought necessary. However, in terms of BLW, which relies on food-handling and chewing skills, they aren't likely to be ready any earlier than their corrected age, which in DS1's case would mean about 8.5 months. This doesn't matter provided you (and everyone around you, including his consultant) are relaxed about this - which is where the nutrition argument comes in:
Full-term babies build up stores of key nutrients in their last few weeks in the womb, so they are born with enough to last them for at least 6 months (probably up to 8 or 9). Preterm babies are born with fewer stores, which may not be enough to last them for 6 months, never mind the additional months it will take them to reach a corrected age of 6 months (i.e., in DS1's case another 2.5 months). In the past, this concern has led to early introduction of solids for preterm babies. We now know that the nutrients they are likely to be short of are those that DS1 is receiving as supplements [iron, folic acid, multi-vitamin], and that breastmilk (or formula) is the best source for everything else, so early solids shouldn't be necessary.
Finally, you may be aware that there are lots of differences of opinion about weaning amongst health professionals. For example, not all of them are in agreement about the '6-months rule' for full term babies - and with so little evidence to draw on, their views on what is appropriate for preterms can vary enormously. Sad to say, many are also influenced by the companies that manufacture baby food. So, while I think you should be guided by what DS1's consultant has to say, it might be good to ask him on what he bases his suggestions, so that you can decide whether or not you want to follow them.