Welcome to the forum
So the vet has confirmed that this piggy is just a young baby and not that there is anything medically wrong?
If so, then he should grow in his own time with a good hay based diet (unless there are health issues).
If there are health issues with him then you must step in with syringe feeding him as well as seeing if there is an actual diagnosis for any illness.
(At his weight it is correct that he should have been removed from his mum - baby boys can get their mothers pregnant from 3 weeks of age or 250g).
1 Weight and Weight Loss
- Why regular weight monitoring matters
- How weight changes over a lifetime
- How to weigh on your kitchen scales (with video)
- The weight loss rules
- How critical is the weight loss for my piggy?
- Possible causes for weight loss
2 Body Mass Index (BMI) or 'Heft'
- Why is understanding your piggy's weight so important?
- 'Average' weight vs. individual weight - the big trip up
- How to check for the BMI...
Introduction
1 Choosing the right place to medicate/feed your piggy
2 Guinea pig whispering and asserting your authority
3 Recovery products and emergency improvisation
4 Syringe recommendations
5 Weight monitoring: your biggest ally
6 Weight loss guidelines and when to step in with feeding
7 Syringe feeding amounts/frequency advice
8 Practical medication and syringing tips
9 Medicating and feeding cooperative guinea pigs (videos)
10 Medicating and feeding uncooperative guinea pigs (hold pictures and tips)
11 The line between life and...
Other factors to consider going forward - spinach is too high in calcium so should only be fed once a week.
As you mention three piggies and mention ‘he’, I assume all piggies are male?
In that event, please prepare that they may not be able to remain together permanently. Sadly boar trios are highly unstable and usually end with fights and separations.
If they are ok for now and assuming the little one isnt being subjected to bullying (not allowed to eat by the other two, loses weight etc) then leave them together for now but as I say, prepare that it unfortunately may not last.
Boar trios require a huge amount of space - a cage measuring 300x100cm at least.
1 Singles and Pairs
Singles: Challenges and solutions for different singles situations (link)
Pairs: A closer look all possible pairings (link)
2 Trios and More by Gender
Boars
- Trios and quartets - Why better avoided
- Large Groups...
1 The recommended ratio of food groups
2 Hay and fresh grass
3 Vegetables, fresh herbs and fruit with an illustrated balanced sample diet
4 Special dietary needs
- Urinary tract infections, bladder stones and sterile IC (non-bacterial interstitial cystitis)
- Diabetes and long term digestive problems
- Impaction in boars
- Pregnancy and nursing dietary tweaks (only visible to registered members who have accepted our no intentional breeding policy)...