I have to agree that it is awful advice from the HV.
As you know I've had problems with Aaron gaining weight too, but the goal has always been to get more milk into him, and not solids (and that was advice from a paediatric dietician)
In all honesty - baby food/purees etc don't contain enough nutritional content to allow weight gain, so I would be tempted to try feeding more milk?
Have you been referred to a dietician yet? I really think you need more help / professional advice - as a lot of HV don't have a clue.
Out of interest - how bad has the weight gain been? Where is bubs on the centile chart - how much have they fallen?
On the DS chart, Aaron started on 98th centile and stayed there for a while - then he started slowly falling down the chart, till he got to 50th. Then he seems to have settled between 50-75th (believe its about the 9th centile on regular chart). I'm assured that this is very normal for babies to do.