Keeping in mind that this is often due to a 'skinny phase' that happens when a Boxer matures from puppy to adult, there are some steps you should take. The main goal will be to maintain or improve his/her diet - with a slight increase of calories but not quality - and keep the Boxer as healthy as possible while waiting for the skinny stage to end.
1) Reevaluate what you are feeding your Boxer. While a Boxer should be fed a super high quality food at all times, when he/she is underweight and trying to fill out into his adult frame, this is more important than ever. A cheap food with fillers will be brutal on a Boxer that is in his/her skinny phase.
You don't want to try and force a Boxer dog to eat more food than he already does and most Boxers that are too skinny are already eating you out of house and home… So the goal will be to offer the same amount while offering more calories
via wholesome, healthy ingredients. This can be accomplished by adding one or more of the following to meals:
• White water-packed tuna
• Cottage cheese
• And/or a whole egg (with the egg, you may opt to add the shell as well - If you do, you'll want to place the shell pieces into a sandwich bag and use a rolling pin to crush it into fine pieces)
While rice and pastas have their place in a healthy diet (carbs are needed for energy), you'll want to avoid adding any extra rice/pasta/breads since they will bulk up the food but won't offer as many protein-rich calories as the foods listed above.
A scoop of peanut butter -eaten off a wooden spoon- or stuffed into a Kong along with fruit & vegetable pieces and banana slices is a great, healthy snack. As a side note, be sure to use a proper toy cleaner to scrub out the gook and gunk that will accumulate inside Kongs and other treat-release toys.
Homemade Snack to Help Boxer Dog Gain Weight: Since this involves routinely giving a mix of calorie dense food that is higher in fat than recommended for canines, run this by your vet first. Please note that these are not
for meals. Feeding this for full meals could lead to upset stomach, diarrhea and very fast, unhealthy weight gain.
Cook 1 pound of ground beef (70% lean), 8 scrambled eggs, 3/4 cup of ground flaxseed, 1 cup cooked oatmeal (plain), 8 ounces of cottage cheese, 1 cup of peanut butter and 1 tablespoon of molasses. Mix very well. Form into teaspoon sized balls. Please on a cookie sheet and freeze. Once frozen, store in airtight containers like Tupperware. Give thawed balls as snacks. 1 to 3 per day.