I have looked at your other thread, theanswer....I can't see why your HV is concerned, on what you have written there.
Yes, on the new charts he has gone from 25th to 9th and this would be a reason to check all is well - it is not a diagnosis of anything or an alarm bell, but simply a sign for whoever is looking at the chart to make sure the baby is healthy, the mum is feeding often enough (no daft scheduling, for instance).
There is no 'poo chart' I'm aware of for a baby of this age - am I missing something, luciemule?
A baby of this age would not be dehydrated - not without showing serious signs of being ill.
Expressing is not a good idea at all - tells you nothing useful.
Fullness of breasts is a poor guide, too.
And checking for tongue tie - why? This is a baby of 5 mths - no
indication that he is tongue tied, is there?
If you want to give your baby more calories, OP, or at least to offer them, then feeding more often, and giving more 'sides' on each occasion, would be the easiest way. Solids would not be a massive 'risk' but I don't see the advantage of them - they are unlikly to inc. calorie intake better than more breastfeeding